Sunan an-Nasa'i 4340
It was narrated that Anas said:
"The Messenger of Allah reached Khaibar in the morning, and they came out to us carrying their shovels. 
When they saw us they said:  'Muhammad and the army!' And they rushed back inot the fortress. The Messenger of Allah raised his hands, then he said: ' Allahu Akbar, Allahu Akbar, Khaibar is destroyed. Verily, when we descend in field of a people (i.e. near to them), evil will be the morning for those who had been warned! Acquired some donkeys there and we cooked the., Then the caller of the Prophet called out: 'Allah and His Messenger forbid you to eat the flesh of donkeys, for it is an abomination."'
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ يَزِيدَ، قَالَ حَدَّثَنَا سُفْيَانُ، عَنْ أَيُّوبَ، عَنْ مُحَمَّدٍ، عَنْ أَنَسٍ، قَالَ صَبَّحَ رَسُولُ اللَّهِ صلى الله عليه وسلم خَيْبَرَ فَخَرَجُوا إِلَيْنَا وَمَعَهُمُ الْمَسَاحِي فَلَمَّا رَأَوْنَا قَالُوا مُحَمَّدٌ وَالْخَمِيسُ ‏.‏ وَرَجَعُوا إِلَى الْحِصْنِ يَسْعَوْنَ فَرَفَعَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَدَيْهِ ثُمَّ قَالَ ‏
"‏ اللَّهُ أَكْبَرُ اللَّهُ أَكْبَرُ خَرِبَتْ خَيْبَرُ إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ‏"‏ ‏.‏ فَأَصَبْنَا فِيهَا حُمُرًا فَطَبَخْنَاهَا فَنَادَى مُنَادِي النَّبِيِّ صلى الله عليه وسلم فَقَالَ إِنَّ اللَّهَ عَزَّ وَجَلَّ وَرَسُولَهُ يَنْهَاكُمْ عَنْ لُحُومِ الْحُمُرِ فَإِنَّهَا رِجْسٌ ‏.‏