Musnad Abu Ya'la al-Mawsili 105:40

Abdullah ibn Umar ibn Aban narrated to us, Ibn Abi Ghunyah narrated to us, from Uqbah ibn al-Mughirah al-Shaybani, from one who narrated to him, from the grandfather of his father, al-Mukhariq, who said: I met Ammar ibn Yasir on the day of the Camel while he was urinating in a horn. I said to him, "Shall I fight with you and be with you?" He said, "Fight under the banner of your people, for the Messenger of Allah, peace and blessings of Allah be upon him, used to like for a man to fight under the banner of his people."