Ahl al-Bayt (a) (Arabic: أهل البیت) means members of a household or a family. In Islamic terminology it applied for some members of the family of the Holy Prophet (s). The term Ahl al-Bayt have appeared in hadiths in different senses, however, in Shi'a literature, it is used to refer to Imam 'Ali (a), Lady Fatima (a), Imam al-Hasan (a) and Imam al-Husayn (a) and the nine infallible Imams (a) who were the children of Imam al-Husayn (a). The Holy Qur'an has used this term in the al-Tathir and al-Mawadda verses. The Holy Prophet (s) has stated the high position of Ahl al-Bayt in Islam in such hadiths as Hadith al-Safina and Hadith al-Thaqalayn.

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From the Shi'a point of view, Ahl al-Bayt (a) have a pivotal position in religion and they are infallible and superior to all other companions of the Prophet (s) in regards to taqwa (God-wariness) and other virtues. Loving them is mandatory upon all Muslims. According to Shi'a teachings, Ahl al-Bayt (a) have guardianship and leadership upon Muslims and Muslims must regard them as their own authorities and refer to them in their religious issues.

Lexical Analysis
In Arabic lexical references, the word "ahl" (أهل) implies a kind of relation and connection between a person with another. For example, in Arabic, wife is known as the "ahl" of her husband, people of every prophet is known as his "ahl" or the residents of a city are the "ahl" of that city. Also, followers of a religion are the "ahl" of that religion.Ahl al-Bayt (a) means the people of the house of the Prophet (s); The famous lexicographer, Ibn Hayyan, has defined Ahl al-Bayt as :"The family of a man: his wife and closely related people to him." This title has a special meaning among Muslims.The word "آل" (Āl) has also been "أهل" (ahl) the letter "ha" (ه) in it has first turned into hamza and then into alif.The usage of the word "āl (آل)" is narrower than the usage of "ahl" because "آل" has never been added to an adverb of place, time, etc. and collocates only with human being, and even to certain people such as Ibrahim (a) in Al Ibrahim (a), 'Imran in Al 'Imran, and to Fir'awn (Pharaoh) in Al Fir'awn.

In the Qur'an
"Ahl al-Bayt" has been mentioned in the Qur'an in three places:Verse 73 of Sura Hud, where it refers to Prophet Abraham (a) and his wife, "They said, 'Are you amazed at Allah's dispensation? [That is] Allah's mercy and His blessings upon you, members of the household. Indeed He is all-laudable, all-glorious.'" (Qur'an 11:73). Verse 12 of Sura al-Qasas which refers to the family of Prophet Moses (a), "We had forbidden him to be suckled by any nurse since before. So she (Moses' sister) said, 'Shall I show you a household that will take care of him for you and will be his well-wishers?'" (Qur'an 28:12). athir verse, where God addresses the Prophet (s) and his family, "Indeed Allah desires to repel all impurity from you, O Ahl al-Bayt, and purify you with a thorough purification." (Qur'an 33:33). About the question over who is meant in this verse by the "Ahl al-Bayt", there are different opinions. An opinion agreed by Shi'a and many Sunni scholars is that it refers to the Ahl al-Kisa', i.e. the Prophet (s), 'Ali (a), Fatima (a), al-Hasan (a) and al-Husayn (a).

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Infallibility
The most distinguished merit of the Ahl al-Bayt (a) is their infallibility which can clearly be understood from the al-Tathir verse, since in this verse, Ahl al-Bayt (a) are referred to as those whom God desired to repel all impurity from.The word "إنما" ["Innama"] in this verse [al-Tathir] and the narrations about the cause of its revelation shows that this [being free from all impurities] is among the merits of the Ahl al-Bayt (a) and it is unique to them. Hadith al-Thaqalayn is also among mutawatir [frequently narrated] hadiths which there is no doubt in its reference and it proves the infallibility of the Ahl al-Bayt (a) of the Prophet (s) (in its very special usage), because in this hadith, Ahl al-Bayt (a) have come as al-Thiql al-Asghar [the smaller weight] together with the Qur'an which is al-Thiql al-Akbar [the greater weight] and they have been considered as the two precious legacies of the Prophet (s) which would never be separate from each other; and if Muslims adhere to them, they will never be misguided. Indeed the Holy Qur'an is the word of God and no error is in it, "falsehood cannot approach it, from before it nor from behind it." (Q 41:42)Therefore, the Ahl al-Bayt (a) of the Prophet (s) who are positioned alongside the Qur'an, are free from error and adhering to them prevents going astray from the right path.According to some Sunni scholars, there is no doubt in the moral and practical infallibility of the Ahl al-Bayt (a) who include lady Fatima (a) and the twelve Imams (a) of Shi'a, and only an ignorant person who rejects Islam would have doubt in it's truth. Instead, what is disagreed upon is their infallibility in knowledge;however because the hadith al-Thaqalayn regards adherence to the Ahl al-Bayt (a) in religion as keeping one safe from ignorance and going astray, their Infallibility in knowledge can easily be understood, per the Shia view.

Superiority of Ahl al-Bayt (a)
From the hadith al-Thaqalayn, superiority of the Ahl al-Bayt (a) over others can easily be understood, because the Prophet (s) put them beside the Qur'an, called the Qur'an as "al-Thiql al-Akbar" and Ahl al-Bayt (a) as "al-Thiql al-Asghar" and never put another person beside the Qur'an. Therefore, the Ahl al-Bayt (a) are superior to others the same way as the Holy Qur'an is superior over other books.The al-Mubahala verse implies the superiority of Ahl al-Kisa' over other companions of the Prophet (s) as well; since, according to this verse, God has given the Prophet (s) mission to choose some people from among children, Muslim men and women to accompany him in Mubahala with the Christians of Najran; and the Prophet (s) chose Imam 'Ali (a), Lady Fatima (a) and al-Hasan (a) and al-Husayn (a).Superiority of the Ahl al-Bayt (a) can also be understood from other hadiths and verses such as the al-Mawadda verse (42:23), hadith al-Safina, hadith Bab al-Hitta, hadith al-Nujum, etc.Most of Shi'as believe that infallible imams are superior to previous prophets except the Prophet Muhammad (s). Many hadiths support this belief. The Qur'an considers imamate superior to prophethood,as Ibrahim (a) first was a prophet and then after some trials, God assigned him as Imam.

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Authority in Knowledge
In the Hadith al-Thaqalayn
Hadith al-Thaqalayn implies the authority of the Ahl al-Bayt (a) in knowledge, because the Prophet (s) asked Muslims to adhere to the Qur'an and his Ahl al-Bayt (a) so that they do not go astray.The first and most essential knowledge reference and authority of Muslims is the glorious Qur'an and after that the tradition of the Prophet (s). Regarding this, the role of the Ahl al-Bayt (a) is that they are interpreters of the Qur'an, guardians and transmitters of the tradition of the Prophet (s). Surely, the Prophet (s) explained a great part of the facts and teachings of the Qur'an for people, but he (s) left another part the conditions for explaining which were not provided or it was more appropriate for them to be explained later for his infallible Ahl al-Bayt (a).Thus, full guardianship of what the Prophet (s) had explained, and explaining what was not explained was given to the Ahl al-Bayt (a).The true way of knowing the Qur'an and the tradition of the Prophet (s) is adhering to the Ahl al-Bayt (a) of the Prophet (s), thus they are the knowledge authority of Muslims for knowing religious teachings and rulings.
The Sunni scholar, Mulla 'Ali al-Qari, said, "Ahl al-Bayt [people of the house] are usually more knowledgeable about the owner of the house and his state; therefore, the Ahl al-Bayt (a) are the most knowledgeable ones to the conduct of the Prophet (s), his manner, his rules and wisdom and that is why they deserve to stand beside the Book of God Almighty."

In the al-Tathir Verse
In the al-Tathir verse (Quran 33:33), the glorious Qur'an introduces the Ahl al-Bayt (a) as those whom God has desired to repel all impurity from and also has mentioned that sublime facts and teachings hidden in the Qur'an cannot be found except by those purified from all impurity. Just the same way that the physical purity is required for touching the text of the Qur'an, purity of the soul from vices is required for understanding the teachings of the Qur'an and the more delicate and deeper the teachings of the Qur'an are, the higher purity of souls are needed to understand them and the greatest of them require infallibility.Therefore, teachings of the Qur'an are not fully and deeply known by anyone other than the Prophet (s) and the infallible ones (a) and thus to learn them, one needs to refer to them.Imam al-Sadiq (a) said, "The facts about the past and future and rulings which distinguish the truth and falsehood about the life of human being are mentioned in the Qur'an and we know them." There are many hadiths narrated from Imams (a) in this regard.

Necessity of Following the Ahl al-Bayt (a)
The necessity of following the Ahl al-Bayt (a) can be inferred from the hadith al-Thaqalayn, because in this hadith, salvation of the Muslim world from ignorance is bound to adherence to the Book of God and the Ahl al-Bayt (a) of the Prophet (s). Adherence to the Qur'an is in fact knowing its instructions and following them. Adherence to the Ahl al-Bayt (a) is the same; i.e. first their orders need to be learned and then they need to be obeyed.

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In the Uli l-Amr Verse
In the Uli l-Amr verse, it is made obligatory for anyone to obey God, His messenger, and Uli l-Amr, "O you who have faith! Obey Allah and obey the Apostle and Uli l-Amr (those vested with authority) among you." (Quran 4:59) Since, Uli l-Amr is added to "the Apostle" without repetition of the verb "ati'u" (أطیعوا, obey), it can be understood that the criteria for obligation of obeying Uli l-Amr are the same as the criteria for obligation of obeying the Prophet (s); and obeying the Prophet (s) is because of his God-given leadership and his infallibility and if he (s) was not infallible, obeying him would not become obligatory without any conditions. The same justification is true about Uli l-Amr, and because of their infallibility, their obedience has been mentioned as absolutely obligatory.Therefore, the Uli l-Amr verse implies the infallibility of those who take the leadership of the Islamic society after the Prophet (s) and also the obligation of their obedience.

In the Hadith al-Safinaa

The hadith al-Safina implies the obligation of following the Ahl al-Bayt (a) as well, since in this hadith, the Prophet (s) has likened his Ahl al-Bayt (a) to the Arc of Noah (a) which anyone who entered it rescued and anyone refused was perished.Ibn Hajar al-Haytami has said that, "the point of likening them [the Ahl al-Bayt (a)] to the Arc of Noah (a) is that anyone who, for the sake of praising the one who has given them such an honor, loves them, respects them and benefits from their guidance and applies it will be saved from darkness and anyone who opposes them will be perished in the sea of ingratitude and gorges of disobedience." About the origin of the hadith of Safina, he said, "This hadith has been narrated from different ways, some of which support the others."

Love of the Ahl al-Bayt (a)
There is no doubt about the obligation of the love of the Ahl al-Bayt (a) of the Prophet (s). In the al-Mawadda verse,
“Say, 'I do not ask you any reward for it except love of [my] relatives'.” Qur'an 42:23
the love of the Prophet's (s) relatives is mentioned as the reward for his mission. Qurba [قربی, relatives) in this verse refers to those about whom the al-Tathir verse has been revealed.After quoting the narrations about the necessity of the love of the Ahl al-Bayt (a), Ibn Hajar al-Haytami says, "From previous narrations, the necessity of the love of the Ahl al-Bayt (a) and great prohibition of having grudge towards them can be learned. Al-Bayhaqqi, al-Baghawi and others have expressly mentioned the necessity of loving them and in some verses quoted from him, al-Shafi'i also has mentioned it as he has said, 'O Ahl al-Bayt (a) of the Messenger of Allah! Your love is an obligation from Allah revealed in the Qur'an.'"
Fakhr al-Din al-Razi has explained the necessity of the love of the Ahl al-Bayt (a), "There is no doubt that the Prophet (s) loved 'Ali (a), Fatima (a), al-Hasan (a), and al-Husayn (a). Therefore, it is obligatory for all the Muslims to do so because God has said, 'and follow him so that you may be guided' (Qur'an 7:158) and also told the Prophet (s), "say, 'If you love God, follow me so that God loves you'" (Qur'an 3:31) and said, 'In the Apostle of Allah there is certainly for you a good exemplar.'" (Qur'an 33]:21)
From all the verses about the reward of the Prophet's (s) mission we understand that he (s) asked for no material or non-material rewards from Muslims for himself. Instead, he (s) asked people to love the Ahl al-Bayt (a) as the reward for his mission which actually benefited themselves,
“     Say, Whatever reward I may have asked you is for your own good. My [true] reward lies only with Allah, and He is witness to all things.” Qur'an 34:47

Wilaya and Leadership
There are many intellectual and traditional reasons for the wilaya and leadership of the Ahl al-Bayt (a). From the viewpoint of intellect and according to the philosophy of Imamate, infallibility is one of the most important requirements of an Imam (a). Verses from the Qur'an also support this issue (Cf. Uli l-Amr verse and al-Sadiqin verse)
On the other hand, Ahl al-Bayt (a) are infallible, thus imamate and leadership of the Islamic world after the Prophet (s) is upon them. Moreover, obeying the Ahl al-Bayt (a) is obligatory. The reasons of the obligation of following the Ahl al-Bayt (a) are absolute and includes all obligations and prohibitions of the lives of Muslims and does not make any difference between ritual, economic, political and cultural issues. For example, in the Uli l-Amr verse, obeying the Uli l-Amr has the very same position and wide usage which obeying the Prophet (s) has. As it was explained, Uli l-Amr are infallible as this quality only belongs to the Ahl al-Bayt (a) of the Prophet (s).From the discussions about the love of the Ahl al-Bayt (a), it can be understood that such a love has an introductory role and the ultimate goal is that people find the path of truth and by following it, they reach happiness and salvation. Finding and walking the path of truth includes all individual, social, ritual, and political aspects of life.The important point in this regard is that, in many reports of the hadith al-Thaqalayn, both adherence to the Ahl al-Bayt (a) and the wilaya of Imam 'Ali (a) have been mentioned. In other words, in the occasion of Ghadir Khumm, the Prophet (s) talked both about his Ahl al-Bayt (a) and advised Muslims to follow them and introduced Imam 'Ali (a) as the wali and leader of the Islamic Umma after himself.Doing so, the Prophet (s) showed that wilaya and leadership of Imam 'Ali (a) is the first step for actualizing the message of hadith al-Thaqalayn.Another point is that in some reports of the hadith al-Thaqalayn, the Qur'an and the Ahl al-Bayt (a) have been referred to as "khalifatayn", "Indeed, I have left among you two caliphs: the Book of God and my Ahl al-Bayt (a)." According to this hadith, Ahl al-Bayt (a) are successors of the Prophet (s) and their rule is overall and encompassing.
In another hadith, after mentioning that the Prophet (s) introduced the Qur'an and the Ahl al-Bayt (a) as the two important legacies he left for the Islamic world, it is mentioned that, "the earth will never be empty of the Ahl al-Bayt (a), otherwise, it will become furious over its dwellers." Then he (s) said "O God! You will never leave the earth empty from Your "hujjas".They are few in number and great in position before You."Therefore, Ahl al-Bayt (a) are the hujjas of God on earth, and imamate and leadership belong to them. Another evidence for the fact that hadith al-Thaqalayn implies the imamate of the Ahl al-Bayt (a) is that Imam 'Ali (a) has referred to it as proof and evidence in different occasions, one of which was on the Day of Shawra.Further evidence referring to this point is Imam Ali's (a) debate with Talha, 'Abd al-Rahman b. 'Awf, and Sa'd b. Abi Waqqas as well as his debate at the time of the caliphate of 'Uthman in the mosque of the Prophet (s) in front of a gathering of companions of the Prophet (s).Ahmad b. Hanbal narrated from Abu Hurayra that the Prophet (s) looked at 'Ali (a), Fatima (a), al-Hasan (a) and al-Husayn (a) and said, "I am at war with anyone who is at war with you and at peace with anyone who is at peace with you." From this narration, it can be inferred that obeying them is obligatory.