Musannaf Ibn Abi Shayba 28:1075

'Abdullah ibn Idris narrated to us, from Muhammad ibn Ishaq, from 'Abdullah ibn Abi Bakr, from 'Amrah, who said: The banner of the Messenger of Allah (may Allah bless him and grant him peace) was black, made from a *mirṭ* (a garment) of 'Aishah that had markings on it.