Sahih Bukhari Volume 5, Book 64, Hadith 4222

حَدَّثَنَا سَعِيدُ بْنُ سُلَيْمَانَ، حَدَّثَنَا عَبَّادٌ، عَنِ الشَّيْبَانِيِّ، قَالَ سَمِعْتُ ابْنَ أَبِي أَوْفَى ـ رضى الله عنهما ـ أَصَابَتْنَا مَجَاعَةٌ يَوْمَ خَيْبَرَ، فَإِنَّ الْقُدُورَ لَتَغْلِي ـ قَالَ وَبَعْضُهَا نَضِجَتْ ـ فَجَاءَ مُنَادِي النَّبِيِّ صلى الله عليه وسلم لاَ تَأْكُلُوا مِنْ لُحُومِ الْحُمُرِ شَيْئًا وَأَهْرِيقُوهَا‏.‏ قَالَ ابْنُ أَبِي أَوْفَى فَتَحَدَّثْنَا أَنَّهُ إِنَّمَا نَهَى عَنْهَا لأَنَّهَا لَمْ تُخَمَّسْ‏.‏ وَقَالَ بَعْضُهُمْ نَهَى عَنْهَا الْبَتَّةَ، لأَنَّهَا كَانَتْ تَأْكُلُ الْعَذِرَةَ‏

Narrated By Ibn Abi Aufa : We where afflicted with severe hunger on the day of Khaibar. While the cooking pots were boiling and some of the food was well-cooked, the announcer of the Prophet came to say, "Do not eat anything the donkey-meat and upset the cooking pots." We then thought that the Prophet had prohibited such food because the Khumus had not been taken out of it. Some others said, "He prohibited the meat of donkeys from the point of view of principle, because donkeys used to eat dirty things."