Jami` at-Tirmidhi 1680

Narrated Yunus bin 'Ubaid: the freed salve of Muhammad bin Al-Qasim said: "Muhammad bin Al-Qasim sent me to Al-Bara' bin 'Azib to ask him about the flag of the Messenger of Allah (ﷺ). He said: 'It was a black square of Namirah.'" [Abu 'Eisa said:] There are narrations on this topic from 'Ali, Al-Harith bin Hassan, and Ibn 'Abbas. [Abu 'Eisa said:] This Hadith is Hasan Gharib, we don know know of it except from the report of Ibn Abi Za'idah. And Abu Ya'qub Ath-Thaqafi's name is Ishaq bin Ibrahim. 'Ubaidullah bin Musa also reports from him