Musnad Ahmad 28:451

Yazid ibn Harun narrated to us, Al-Awwam informed us, Aswad ibn Mas'ud narrated to me, from Hanzalah ibn Khuwaylid al-Anazi, who said: While I was with Mu'awiyah, two men came to him disputing over the head of Ammar, each of them saying: "I killed him." Abdullah ibn Amr said: "Let one of you be happy with it for his companion, for I heard the Messenger of Allah (peace be upon him) — [Abdullah ibn Ahmad said: Thus my father said: meaning the Messenger of Allah (peace be upon him)] — saying: 'The transgressing group will kill him.'" Mu'awiyah said: "Will you not rid us of your madman, O Amr? Why are you with us?" He said: "My father complained about me to the Messenger of Allah (peace be upon him), and the Messenger of Allah (peace be upon him) said to me: 'Obey your father as long as he is alive, and do not disobey him.' So I am with you, but I am not fighting."