Musnad Ahmad 31:143

Abu al-Nadr narrated to us, Muhammad—meaning Ibn Talhah—narrated to us, from al-A’mash, from ‘Atiyyah al-‘Awfi, from Abu Sa’id al-Khudri, from the Prophet (peace and blessings of Allah be upon him), who said: "I am about to be called, and I will answer. I am leaving among you the two weighty things: the Book of Allah—Mighty and Majestic is He—and my progeny. The Book of Allah is a rope extended from the heaven to the earth, and my progeny is my household (Ahl al-Bayt). The Subtle, the All-Aware has informed me that they will never separate until they return to me at the Basin. So, look at how you will succeed me regarding them."