Musnad Ahmad 830:75

Hashim narrated to us, al-Mubarak narrated to us, al-Hasan narrated to us, Abu Bakrah narrated to us, he said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray with the people, and al-Hasan ibn Ali (may Allah be pleased with them both) would jump onto his back when he prostrated. He did that more than once. They said to him: "By Allah, you are doing something with this one that we have not seen you do with anyone else." Al-Mubarak said: He mentioned something, then he said: "Indeed, this son of mine is a leader, and through him, Allah will reconcile two groups of the Muslims." Al-Hasan said: By Allah, by Allah, after he took charge, he did not shed even the amount of a cupping glass of blood during his caliphate.