Musnad Ahmad 410:1

Abdullah ibn Ahmad said: Abu Musa al-Anazi Muhammad ibn al-Muthanna narrated to me, saying: Muhammad ibn Abi Adi narrated to us, from Ibn Awn, from Kulthum ibn Jabr, who said: We were in Wasit al-Qasab with Abd al-A’la ibn Abd Allah ibn Amir. He said: There was a man with him called Abu al-Gadiyah. He asked for water, and he was brought a vessel plated with silver, but he refused to drink from it. He mentioned the Prophet, peace and blessings of Allah be upon him, and mentioned this hadith: "Do not return after me as disbelievers or misguided"—Ibn Abi Adi was in doubt—"striking the necks of one another." Then there was a man insulting so-and-so, so I said: "By Allah, if Allah gives me power over you in a battalion..." When the day of Siffin came, I was with him, and he was wearing armor. He said: I noticed the opening in the collar of the armor. I stabbed him and killed him, and behold, it was Ammar ibn Yasir. He said: I said: "What a hand! It refrained from drinking from a silver-plated vessel, yet it killed Ammar ibn Yasir."