صحيح البخاري

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."