Sahih Muslim 1939
Sahih Muslim · Book 34: The Book of Hunting, Slaughter, and what may be Eaten · The prohibition of eating the meat of domesticated donkeys
Ibn 'Abbas reported:
وَحَدَّثَنِي أَحْمَدُ بْنُ يُوسُفَ الأَزْدِيُّ ، حَدَّثَنَا عُمَرُ بْنُ حَفْصِ بْنِ غِيَاثٍ ، حَدَّثَنَا أَبِي ، عَنْ عَاصِمٍ ، عَنْ عَامِرٍ ، عَنِ ابْنِ عَبَّاسٍ ، قَالَ لاَ أَدْرِي إِنَّمَا نَهَى عَنْهُ رَسُولُ اللَّهِ صلى الله عليه وسلم مِنْ أَجْلِ أَنَّهُ كَانَ حَمُولَةَ النَّاسِ فَكَرِهَ أَنْ تَذْهَبَ حَمُولَتُهُمْ أَوْ حَرَّمَهُ فِي يَوْمِ خَيْبَرَ لُحُومَ الْحُمُرِ الأَهْلِيَّةِ .
I do not know whether Allah's Messenger (ﷺ) prohibited (the eating of the domestic ass) due to the fact that they were the beasts of burden for the people, so he (the Holy Prophet) did not like their beasts of burden to be destroyed (as a matter of expediency), or he prohibited the use of the flesh of domestic asses (not as an expediency but as a law of the Shari'ah) on the Day of Khaibar.