ENOfficial
Narrated Simak ibn Harb: Simak reported on the authority of a man from his people, on the authority of another man from them: I saw that the standard of the Messenger of Allah (ﷺ) was yellow.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عُقْبَةُ بْنُ مُكْرَمٍ، حَدَّثَنَا سَلْمُ بْنُ قُتَيْبَةَ الشَّعِيرِيُّ، عَنْ شُعْبَةَ، عَنْ سِمَاكٍ، عَنْ رَجُلٍ، مِنْ قَوْمِهِ عَنْ آخَرَ، مِنْهُمْ قَالَ رَأَيْتُ رَايَةَ رَسُولِ اللَّهِ صلى الله عليه وسلم صَفْرَاءَ .
sunnah.com