Musannaf Ibn Abi Shayba 27:448

Ubayd Allah narrated to us, from Talha ibn Jubayr, from al-Muttalib ibn Abd Allah, from Mus'ab ibn Abd al-Rahman, from Abd al-Rahman ibn Awf, who said: When the Messenger of Allah (may Allah bless him and grant him peace) conquered Mecca, he departed to al-Ta'if and besieged it for seventeen or eighteen [days], but he did not conquer it. Then he departed at the end of the day or at the beginning of the day, and he camped, then he traveled, then he said: "O people, I am a forerunner for you, and I enjoin you to treat my family (itrah) well. Your appointed meeting place is the Basin (al-Hawd). By the One in whose hand is my soul, you shall surely establish the prayer and you shall surely give the zakat, or I shall surely send to you a man from me—or from myself—who shall strike the necks of their fighters and take their offspring captive." He said: The people thought that it was Abu Bakr or Umar. Then he took the hand of Ali and said: "This [is the one]."