Yunus ibn Ubayd, client of Muhammad ibn al-Qasim, said that Muhammad ibn al-Qasim sent to al-Bara' ibn Azib to ask him about the standard of the Messenger of Allah ﷺ. He said: It was black and square, being made of a woollen rug. (Using translation from Abū Dāʾūd 2591)
عَنْ رَايَةِ رَسُولِ اللهِ ﷺ مَا كَانَتْ؟ فَقَالَ كَانَتْ سَوْدَاءَ مُرَبَّعَةً مِنْ نَمِرَةٍ