"The word ahl al-bayt occurs twice in the Koran: "The mercy of God and His blessing are on you, O people of the house, ..." (11:73). This verse refers to the people of the house of Abraham (Kashf al-Asrar wa Uddat al-Abrar, 4:416), and to the house of the Prophet Muhammad (peace be upon him): "God desires only to keep away the uncleanness from you, O people of the House! And to purify you a (thorough) purifying (33:33)".
The ahl al-bayt are those glowing personalities, endeared and respected by every Muslim alike who loves the Prophet. The Muslims have known this lofty term, the guiding star on the horizon of Islam ever since the Koran pronounced this blessed epithet, to address the family of the Prophet.
The word ahl (pl. ahalil) is derived from the Hebrew, ohel, means those who occupy with one the same tent, the family or inmates, relative, folk, kin, kinfolk, family, people, members, followers, etc. Under these meanings, the word ahl occurs 31 times in the Koran. According to another view, it is derived from ahala, meaning to demonstrate (the family). The term ahl signifies the members of one's household, including his fellow tribesmen, kin, relatives, wife (or wives), children, and all those who share a family background, religion, housing, city, and country with him. The term Al, meaning nearer or nearest relations by descent from the same father or ancestor or a man's family or kinsman, is used 26 times in the Koran in connection with the descendants of the prophets or those who succeeded them. The Koran says: "Verily, God has chosen Adam and Noah, the family of Abraham (Al Ibrahim) and the family of Imran (Al Imran) above all people" (3:33). In another verse (4:54), we read: "Or do they envy the people for what God has given them of His grace: But indeed We have given to Abraham's children (Al Ibrahim) the Book and the Wisdom and gave them a great kingdom".
Ahl and Al are both the same term with the exception that Al is exclusively used for human beings and should come before the family name, but such a condition is not existent in the case of Ahl. Ahl is used in a broader sense in referring to the people of a town or inhabitation, a group or followers. When used in conjunction with the term bayt (ahl al-bayt), it refers to the immediate descendants of a family or such a family of the same house (bayt).
The word bayt (pl. buyut) is used in the Koran in fifteen instances to denote the house of God, which is described variously as "the first house," "the ancient house," "the sacred house," "the forbidden house," "the freaquented house" and "My house." Only once, however is it identified explicitly with the Kaba (5:97) and twice with the "Sacred Mosque" (5:2, 8:34-5). Indeed, the Koran uses the term bayt more frequently to designate a holy place, habitation and dwelling including tents and buildings both. "The Ahl al-Bayt of any person refers to his family members and all those who live in his house" (Mufradat al-Koran by Raghib).
In this compound form, Ahl al-Bayt is used in the Koran especially in reference to the immediate family of the Prophet. The Koran says: "God desires only to keep away the uncleanness from you, O people of the House! And to purify you a (thorough) purifying (33:33)". The commentators of the Koran are unanimous in their opinion that the term Ahl al-Bayt in this verse refers to the Prophet's daughter Fatima, his cousin and son-in-law Ali, and his grandsons Hasan and Hussain.
Zayd bin Arqam states that the wives of the Prophet are not regarded as the members of his Household. He was asked: "Aren't the wives of the Prophet considered as the members of the Household?" He replied: "The wives of the Prophet reside in the Prophet's house but the Prophet's ahl al-bayt are those to whom the grant of sadaqa is religiously unlawful." Another tradition has it that Zayd was asked whether the Prophet's wives were among his ahl al-bayt. He replied: "No, a wife lives with a husband for a while and then might be divorced and go back to her parents."