Musannaf Ibn Abi Shayba 36:419

Ubayd Allah ibn Musa narrated to us, from Talha ibn Jabr, 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 (peace and blessings of Allah be upon him) conquered Mecca, he departed to al-Ta'if and besieged them for nineteen or eighteen [days], but he did not conquer it. Then he departed for a journey of an evening or a morning, then he stopped. Then he said: "O people, I am a forerunner for you, and I advise you to treat my family well. Your appointed meeting place is the Basin. By the One in whose hand is my soul, they shall surely establish the prayer and give the zakat, or I shall surely send to them a man from me—or like 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 one."