Musnad Abu Ya'la al-Mawsili 42:54

And from Abu Sa'id, from the Prophet, may Allah bless him and grant him peace, who said: "I am leaving among you the two weighty things, one of them is greater than the other: the Book of Allah, a rope extended between the heaven and the earth, and my progeny, the people of my house. And they shall not separate until they return to me at the Basin."