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»  

[Machine] "I entered the mosque and found it crowded with people, and I saw a black banner. I asked, 'What's the matter with the people today?' They replied, 'This is the Messenger of Allah ﷺ wanting to send 'Amr ibn Al-'As on a mission.'"  

«دَخَلْتُ الْمَسْجِدَ فَإِذَا الْمَسْجِدُ غَاصٌّ بِالنَّاسِ فَإِذَا رَايَةٌ سَوْدَاءُ» قُلْتُ «مَا شَأْنُ النَّاسِ الْيَوْمَ؟» قَالُوا «هَذَا رَسُولُ اللهِ ﷺ يُرِيدُ أَنْ يَبْعَثَ عَمْرَو بْنَ الْعَاصِ وَجْهًا»