Al-Matalib al-'Aliyah 4417 — The killing of Ammar at Siffin

It is narrated from Hanzalah bin Khuwaylid (may Allah have mercy on him) that he said: I was sitting in the gathering of Sayyiduna Mu'awiyah (may Allah be pleased with him) when suddenly two men came, quarrelling about the head of Sayyiduna Ammar (may Allah be pleased with him) (about obtaining it or about having killed him), and each of them was claiming: "I killed him." Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) said: "One of you should willingly leave it (the responsibility for the killing) to his companion (that is, withdraw from it), for I heard the Messenger of Allah (peace and blessings be upon him) say: 'A rebellious group will kill Ammar.'" Hearing this, Sayyiduna Mu'awiyah (may Allah be pleased with him) said (to Sayyiduna Amr bin al-As, may Allah be pleased with him): "O Amr! Will you not keep this madman of yours away from us? If this is his view, then what is he doing here with us?" He (Sayyiduna Abdullah bin Amr) replied: "Indeed, I am with you, but I am not fighting. The reason is that my father complained about me to the Messenger of Allah (peace and blessings be upon him), and the Messenger of Allah (peace and blessings be upon him) said to me: 'Obey your father as long as he is alive, and do not disobey him.' That is why I am with you but do not take part in the fighting."
The chain of this hadith is sahih (authentic), and Allamah Shu'ayb al-Arna'ut declared it sahih (authentic) in his takhrij of 'Musnad Ahmad'.