Musnad Ahmad 938:3

Al-Aswad ibn Amir 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, a rope extended between the heaven and the earth—or between the heaven to the earth—and my progeny, the people of my house. And indeed, they will not separate until they return to me at the Basin."