I met Ammar on the day of the Battle of the Camel while he was urinating into a horn. I said to him, "Shall I fight alongside you and stand with you?"He replied, "Fight under the banner of your own people, for the Messenger of Allah ﷺ preferred that a man fight under the banner of his own people."
حدثنا يحيى بن عبد الملك بن أبي غنية قال حدثنا عقبة بن المغيرة
لقيت عمارا يوم الجمل وهو يبول في قرن فقلت أقاتل معك فأكون معك قال قاتل تحت راية قومك فإن رسول الله ﷺ كان يستحب للرجل أن يقاتل تحت راية قومه
No references are available.
Its chain of transmission is daif for several reasons, due to the obscurity of Uqba bin Mughira and his father's grandfather Mukharraq, and due to the disconnection between them. Haythami said the same, 5/326.