Al-Sha'bi said: I saw Sayyiduna Hasan bin Ali (may Allah be pleased with him) at al-Nukhaylah when Sayyiduna Mu'awiyah (may Allah be pleased with him) made peace with him. Mu'awiyah (may Allah be pleased with him) said to him: "When this is done, stand up and speak, and tell the people that you have handed this matter over to me." (The narrator Sufyan sometimes said: Tell the people about this matter which you have left to me.) So he stood up and delivered a sermon from the pulpit, praising and extolling Allah - al-Sha'bi said: and I was listening - then he said: "To proceed: the greatest wisdom is piety, and the greatest foolishness is sin. This matter in which Mu'awiyah and I differed was either my right, which I left to Mu'awiyah, desiring the well-being of this Ummah and to spare their blood, or it was the right of someone more entitled to it than me, so I did that. 'And I do not know; perhaps it is a trial for you and an enjoyment for a time.' [Surah al-Anbiya: 111]."
Hadith Referenceالمعجم الكبير للطبراني / باب الحاء / 2546