Muhammad bin Qasim sent me to Bara bin Azib to ask him about the banner of the Messenger of Allah ﷺ, what it was like.He said, "It was black and square, made of a woolen cloth called namirah."
حدثنا يحيى بن زكريا حدثنا أبو يعقوب الثقفي
بعثني محمد بن القاسم إلى البراء بن عازب أسأله عن راية رسول الله ﷺ ما كانت قال كانت سوداء مربعة من نمرة
Its isnad is hasan, because of Abu Yaqub Thaqafi, who is Ishaq bin Ibrahim Kufi. Ibn Hibban deemed him trustworthy, Nasai graded him sahih, and Ibn Main graded him daif. Abu Dawud, Ibn Majah, and Tirmidhi narrated from him, and Tirmidhi graded this hadith of his hasan.The hadith was narrated by Abu Dawud 3/32, no. 2591, in jihad, banners and standards, and by Tirmidhi likewise 4/196, no. 1680, who said, hasan gharib.