The term “Ahlul Bayt” (people of the house) has been repeated twice in the Holy Qur'an:
1. “... the mercy of Allah and his blessing are on you, O people of the house, ... (11:73)”
This verse refers to the people of the House of Ibrahim (S) (c.f. “Kashf al-Asrar wa `Uddat al-Abrar”, 416/4 and other interpretations).
2. “... Allah only desires to keep away the uncleanness from you, O people of the House! And to purify you a (thorough) purifying (33:33)”.
This verse, known as the “Tathir verse”, refers to the Members of the Household of the Holy Prophet (S). The Imamiyyah scholars of hadith and fiqh, as well as some Sunni `Ulama, consider the “Ahlul Bayt” cited in the “tathir verse” to include exclusively Muhammad, `Ali, Fatimah, Hasan, and Husayn (peace be upon them all). They do not consider the Holy Prophet's other offspring, wives, sons of paternal uncles, and dwellers of his house as the Messenger's “Ahlul Bayt”.
They base their argument on the genuine and authentic traditions narrated by the companions of the Prophet (S) recorded in the Sunni and Shi`i sources. some of the said traditions and refer to some features of the “Ahlul Bayt” as narrated by the Sunnis:
Kisa' tradition; Mawaddat al-Qurba tradition; manzalah tradition; thaqalayn tradition; Safinah tradition; Other traditions.
manzalah tradition:
the Prophet said, “‘Ali is in relation to me as Aaron was to Moses, other than that after me there will be no Prophet.”
thaqalayn tradition:
The basic Tradition, according to Abu Dharr Ghifari, is that once when he was in the Ka’bah, he turned to the people and said that I heard that the Holy Prophet had said “I leave two things of value among you: the Qur’an and my family. These two will never separate from each other until they enter unto me in the Fountain of Abundance (kawthar) in Paradise. Thus, take care to follow what I have recommended” (Recorded from Jama’ Tarmjghi as from Niyabi’al-mawadah, p. 37)