From Sahih Bukhari (Hadith 4173):
Narrated Zahir Al-Aslami:(who was one of those who had witnessed (the Pledge of allegiance beneath) the Tree) While I was making fire beneath the cooking pots containing donkey's meat, the announcer of Allah's Messenger (ﷺ) announced, "Allah's Messenger (ﷺ) forbids you to eat donkey's meat
Reference: Bukhari 4173