Donkey meat is considered Haram (forbidden) in Islam primarily because the Prophet Muhammad (ﷺ) declared it impure.
The Prophetic Prohibition
According to a narration, during a certain time, people were facing hardship, and some began slaughtering and eating donkeys. When this news reached the Prophet Muhammad (ﷺ), he instructed a public announcement to be made forbidding the consumption of donkey meat.
As referenced:
Someone came to Allah's Messenger (ﷺ) and said, "The donkeys have been (slaughtered and) eaten. Another man came and said, "The donkeys have been destroyed." On that the Prophet (ﷺ) ordered a caller to announce to the people: Allah and His Apostle forbid you to eat the meat of donkeys, for it is impure.
The Reason Stated
The explicit reason given in this declaration for the prohibition of eating donkey meat is its impurity.
Based on the direct statement from the Prophet (ﷺ) as mentioned in the reference, the fundamental reason for domestic donkey meat being Haram is that it was declared impure.
