It was narrated from Abu Tha''Aabah Al-Khushani that: they went on a military campaign with the Messenger of Allah (ﷺ) to Khaibar, and the people were starving. They found some domesticated donkeys there, so the people slaughtered some of them. The Prophet (ﷺ) was told about that, and he ordered 'Abdur-rahman bin 'Awf to announce to the people: "The flesh of domesticated donkeys is not permissible for the one who testifies that I am the Messenger of Allah."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَمْرُو بْنُ عُثْمَانَ، أَنْبَأَنَا بَقِيَّةُ، عَنْ بَحِيرٍ، عَنْ خَالِدِ بْنِ مَعْدَانَ، عَنْ جُبَيْرِ بْنِ نُفَيْرٍ، عَنْ أَبِي ثَعْلَبَةَ الْخُشَنِيِّ، أَنَّهُ حَدَّثَهُمْ أَنَّهُمْ، غَزَوْا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم إِلَى خَيْبَرَ وَالنَّاسُ جِيَاعٌ فَوَجَدُوا فِيهَا حُمُرًا مِنْ حُمُرِ الإِنْسِ فَذَبَحَ النَّاسُ مِنْهَا فَحُدِّثَ بِذَلِكَ النَّبِيُّ صلى الله عليه وسلم فَأَمَرَ عَبْدَ الرَّحْمَنِ بْنَ عَوْفٍ فَأَذَّنَ فِي النَّاسِ " أَلاَ إِنَّ لُحُومَ الْحُمُرِ الإِنْسِ لاَ تَحِلُّ لِمَنْ يَشْهَدُ أَنِّي رَسُولُ اللَّهِ " .
sunnah.com