Musnad Ahmad 658:155

Yahya ibn Zakariya narrated to us, Abu Ya'qub al-Thaqafi narrated to us, Yunus ibn 'Ubayd, the freed slave of Muhammad ibn al-Qasim, narrated to me, saying: 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, made of namirah (striped garment)."