Sahih Bukhari Volume 5, Book 64, Hadith 4200

أَخْبَرَنَا صَدَقَةُ بْنُ الْفَضْلِ، أَخْبَرَنَا ابْنُ عُيَيْنَةَ، حَدَّثَنَا أَيُّوبُ، عَنْ مُحَمَّدِ بْنِ سِيرِينَ، عَنْ أَنَسِ بْنِ مَالِكٍ ـ رضى الله عنه ـ قَالَ صَبَّحْنَا خَيْبَرَ بُكْرَةً، فَخَرَجَ أَهْلُهَا بِالْمَسَاحِي، فَلَمَّا بَصُرُوا بِالنَّبِيِّ صلى الله عليه وسلم قَالُوا مُحَمَّدٌ وَاللَّهِ، مُحَمَّدٌ وَالْخَمِيسُ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ اللَّهُ أَكْبَرُ خَرِبَتْ خَيْبَرُ، إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ‏"‏‏.‏ فَأَصَبْنَا مِنْ لُحُومِ الْحُمُرِ فَنَادَى مُنَادِي النَّبِيِّ صلى الله عليه وسلم إِنَّ اللَّهَ وَرَسُولَهُ يَنْهَيَانِكُمْ عَنْ لُحُومِ الْحُمُرِ، فَإِنَّهَا رِجْسٌ‏.

Narrated Anas bin Malik: We reached Khaibar early in the morning and the inhabitants of Khaibar came out carrying their spades, and when they saw the Prophet they said, "Muhammad! By Allah! Muhammad and his army!" The Prophet said, "Allahu-Akbar! Khaibar is destroyed, for whenever we approach a (hostile) nation (to fight) then evil will be the morning for those who have been warned." We then got the meat of donkeys (and intended to eat it), but an announcement was made by the announcer of the Prophet, "Allah and His Apostle forbid you to eat the meat of donkeys as it is an impure thing."