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