Musnad Abu Ya'la al-Mawsili 42:167

Sufyan ibn Waki' narrated to us, Muhammad ibn Fudayl narrated to us, from 'Abd al-Malik ibn Abi Sulayman, from 'Atiyyah al-'Awfi, from Abu Sa'id al-Khudri, who said: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "O people, I have left among you that which, if you hold fast to it, you will not go astray after me: the two weighty things. One of them is greater than the other: the Book of Allah, a rope extended from the heaven to the earth, and my progeny, the people of my house. And they shall never separate until they return to me at the Basin."