Musnad Ahmad 28:62

Yazid narrated to us, Al-Awwam informed us, Aswad ibn Mas'ud narrated to me, from Hanzalah ibn Khuwaylid al-Anbari, 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 content with it for the other, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'The rebellious group will kill him.'" Mu'awiyah said: "Then why are you with us?" He replied: "My father complained about me to the Messenger of Allah (peace and blessings of Allah be upon him), and he said: 'Obey your father as long as he is alive, and do not disobey him.' So I am with you, but I am not fighting."