It is narrated from Sayyiduna Zayd bin Thabit (may Allah be pleased with him), who attributes it to the Messenger of Allah (peace and blessings be upon him), that he (peace and blessings be upon him) said: "I have left among you two successors: the Book of Allah and my 'itrah (that is, my Ahl al-Bayt), and the two of them will never separate until they come to me at the Pond (Hawd)."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4903