Musannaf Ibn Abi Shayba 38:98

Abu Usama narrated to us, from Mujalid, from al-Sha'bi, from al-Harith, who said: When Ali returned from Siffin, he knew that he would never possess [the caliphate] again. So he spoke of things he used not to speak of, and he narrated hadiths he used not to narrate. He said, among what he said: "O people, do not dislike the leadership of Mu'awiyah. By Allah, if you were to lose him, you would see heads falling from their shoulders like colocynths."