(38) باب غَزْوَةُ خَيْبَرَ
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."
Reference: Sahih al-Bukhari 4198
In-book reference: Book 64, Hadith 238
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 59, Hadith 510