Sahih Bukhari
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
Volume: 005 / Book Number: 059 / Hadith Number: 491
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 Apostle announced, "Allah's Apostle forbids you to eat donkey's meat."
The same narration was told by Majzaa from a man called Uhban bin Aus who was one of those who had witnessed (the Pledge of allegiance beneath) the Tree., and who had some trouble in his knee so that while doing prostrations, he used to put a pillow underneath his knee.