Narrated Anas:

The Prophet reached Khaibar in the morning, while the people were coming out carrying their spades over their shoulders. When they saw him they said, "This is Muhammad and his army! Muhammad and his army!" So, they took refuge in the fort. The Prophet raised both his hands and said, "Allahu Akbar, Khaibar is ruined, for when we approach a nation (i.e. enemy to fight) then miserable is the morning of the warned ones." Then we found some donkeys which we (killed and) cooked: The announcer of the Prophet announced: "Allah and His Apostle forbid you to eat donkey's meat." So, all the pots including their contents were turned upside down.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا سُفْيَانُ، عَنْ أَيُّوبَ، عَنْ مُحَمَّدٍ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ صَبَّحَ النَّبِيُّ ﷺ خَيْبَرَ وَقَدْ خَرَجُوا بِالْمَسَاحِي عَلَى أَعْنَاقِهِمْ، فَلَمَّا رَأَوْهُ قَالُوا هَذَا مُحَمَّدٌ وَالْخَمِيسُ، مُحَمَّدٌ وَالْخَمِيسُ. فَلَجَئُوا إِلَى الْحِصْنِ، فَرَفَعَ النَّبِيُّ ﷺ يَدَيْهِ وَقَالَ
" اللَّهُ أَكْبَرُ، خَرِبَتْ خَيْبَرُ، إِنَّا إِذَا نَزَلْنَا بِسَاحَةِ قَوْمٍ فَسَاءَ صَبَاحُ الْمُنْذَرِينَ ". وَأَصَبْنَا حُمُرًا فَطَبَخْنَاهَا، فَنَادَى مُنَادِي النَّبِيِّ ﷺ إِنَّ اللَّهَ وَرَسُولَهُ يَنْهَيَانِكُمْ عَنْ لُحُومِ الْحُمُرِ، فَأُكْفِئَتِ الْقُدُورُ بِمَا فِيهَا. تَابَعَهُ عَلِيٌّ عَنْ سُفْيَانَ رَفَعَ النَّبِيُّ ﷺ يَدَيْهِ.