Musnad Ahmad 31:223

Ibn Numayr narrated to us, 'Abd al-Malik—meaning Ibn Abi Sulayman—narrated to us, from 'Atiyyah, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, I have left among you the two weighty things, one of them is greater than the other: the Book of Allah, the Mighty and Majestic, a rope extended from the heaven to the earth, and my progeny, the people of my house. Beware, for they shall not separate until they return to me at the Basin."