It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "I am leaving among you two weighty trusts: the Book of Allah and my family ('itrah). So see how you treat them after me." We asked: Who are his (peace and blessings be upon him) Ahl al-Bayt (household)? He said: "The family of Ali, the family of Ja'far, the family of Aqil and the family of Abbas."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5006