Musnad Ahmad 938:79

Abu Ahmad al-Zubayri narrated to us, Sharik narrated to us, from al-Rukin, from al-Qasim ibn Hassan, from Zayd ibn Thabit, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "Indeed, I am leaving among you two successors: the Book of Allah and my family (Ahl al-Bayt). And indeed, they will not separate until they return to me at the Basin together."