Musannaf Ibn Abi Shayba 24:165

Yahya ibn Adam narrated to us, he said: Ibn ‘Uyaynah narrated to us, from Mujalid ibn Sa’id, from al-Sha’bi, who said: When the peace treaty took place between al-Hasan ibn ‘Ali and Mu’awiyah ibn Abi Sufyan, al-Hasan intended to depart—meaning to Medina. Mu’awiyah said to him, "You shall not leave until you address the people." Al-Sha’bi said: So I heard him on the pulpit. He praised Allah and extolled Him, then said: "As for what follows: Indeed, the most sagacious of sagacity is piety, and the most incapable of incapacity is wickedness. And this matter, which I and Mu’awiyah differed over—until it was mine and I left it for Mu’awiyah, or it was a right that belonged to a man more entitled to it than I—I only did this to spare your blood. And I do not know, perhaps it is a trial for you and an enjoyment for a time."