كتاب الصيد والذبائح وما يؤكل من الحيوان
34  
The Book of Hunting, Slaughter, and what may be Eaten
(5)
Chapter: The prohibition of eating the meat of domesticated donkeys
(5)
باب تَحْرِيمِ أَكْلِ لَحْمِ الْحُمُرِ الإِنْسِيَّةِ ‏‏
Sahih Muslim 1939

Ibn 'Abbas reported:

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.
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 ‏.‏
Reference : Sahih Muslim 1939
In-book reference : Book 34, Hadith 46
USC-MSA web (English) reference : Book 21, Hadith 4774
  (deprecated numbering scheme)