Musnad Ahmad 25:1627

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from 'Uthman al-Jazari, from Miqsam, who said: I do not know it except from Ibn 'Abbas: "The banner of the Prophet, may Allah bless him and grant him peace, was with 'Ali ibn Abi Talib, and the banner of the Ansar was with Sa'd ibn 'Ubadah. And when the killing became intense, the Messenger of Allah, may Allah bless him and grant him peace, would be under the banner of the Ansar."