Al-Qutaybi (Ibn Qutaybah) (may Allah have mercy on him) said: "('Itrah' means) a man's children and his children's children, male and female, and his nearest kinsfolk; and what guides you to this is the saying of Sayyiduna Abu Bakr al-Siddiq (may Allah be pleased with him): 'We are the 'itrah (family) of the Messenger of Allah (peace and blessings be upon him) from which he (peace and blessings be upon him) came forth, and his (peace and blessings be upon him) egg from which he (peace and blessings be upon him) hatched (that is, we are his (peace and blessings be upon him) origin and family).'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوقف / Q11927
Sayyiduna Ma'qil bin Yasar al-Muzani (may Allah be pleased with him) says: I heard Abu Bakr al-Siddiq (may Allah be pleased with him) say that Ali bin Abi Talib (may Allah be pleased with him) is the family ('itrah) of the Messenger of Allah (peace and blessings be upon him).
The wording of one narration is that Sayyiduna Abu Bakr (may Allah be pleased with him) said on the day of Saqifah: We are the family ('itrah) of the Messenger of Allah (peace and blessings be upon him).
This chain contains some narrators who are unknown (majhul).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الوقف / 11927