Musnad Abu Ya'la al-Mawsili 7:26

Abu Bakr ibn Abi Shaybah narrated to us, Ubayd Allah ibn Musa narrated to us, from Talhah, 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 be upon him) conquered Makkah, he departed toward al-Ta'if and besieged it for nineteen or eighteen [days], but he did not conquer it. Then he traveled deep into the night or the morning, then he descended, then he departed. He said: "O people, I am a forerunner for you, and I advise you to treat my family (itrah) well. Indeed, your appointed meeting place is the Hawd (the Cistern). By the One in Whose hand is my soul, they must establish the prayer and give the zakat, or I will send to them a man from me—or like myself—who will 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 he."