Al-Matalib al-'Aliyah 3922 — The virtues of Ali

It is narrated from Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) that when the Messenger of Allah (peace and blessings be upon him) had conquered Makkah, he (peace and blessings be upon him) set out towards al-Ta'if and kept it under siege for eighteen or nineteen days, but it was not conquered. Then he (peace and blessings be upon him) travelled some distance in a morning or evening stage and halted at a place; then in the heat of midday he gathered the people and addressed them, saying: "O people! I am your forerunner (at the Pond of Kawthar), and I enjoin you to treat my family ('itrah, the Ahl al-Bayt) with kindness and goodness, and your appointed meeting place with me is the Pond of Kawthar. So by the One in whose hand is my soul! You will certainly keep establishing the prayer and keep paying the zakah, or else I will send to you a man who is from me, or who is like my own self; he will strike the necks of their fighters and take their offspring captive." The narrator says: The people thought that perhaps that man would be Sayyiduna Abu Bakr or Sayyiduna Umar (may Allah be pleased with them both), but the Prophet (peace and blessings be upon him) took hold of the hand of Sayyiduna Ali (may Allah be pleased with him) and said: "This is that man."
And Imam Abu Ya'la also narrated it in the same way through the chain of Abu Bakr bin Abi Shaybah.