Musnad Ahmad 31:571

Ibn Numayr narrated to us, 'Abd al-Malik ibn Abi Sulayman narrated to us, from 'Atiyyah al-'Awfi, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have left among you that which, if you hold fast to it, you will never go astray after me: the two weighty things, and 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. Behold, they will never separate until they return to me at the Basin."