Sahih al-Bukhari 4198
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 (ﷺ) they said, "Muhammad! By Allah! 
Muhammad and his army!" The Prophet (ﷺ) 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."
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.‏