Musnad Abu Ya'la al-Mawsili 106:49

Zakariya ibn Yahya narrated to us, Yahya ibn Zakariya narrated to us, he said: Abu Ya'qub al-Thaqafi narrated to me, from Yunus ibn Ubayd, the freed slave of Muhammad ibn al-Qasim, he said: Muhammad ibn al-Qasim sent me to al-Bara' ibn Azib to ask him about the banner of the Messenger of Allah, may Allah bless him and grant him peace: what was it? He said: It was black, square, and made of striped cloth.