Language:
كتاب المغازى
64  
Military Expeditions led by the Prophet (pbuh) (Al-Maghaazi)
(35)
Chapter: The Ghazwa of Al-Hudaibiya
(35)
باب غَزْوَةِ الْحُدَيْبِيَةِ
Sahih al-Bukhari 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."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، حَدَّثَنَا أَبُو عَامِرٍ، حَدَّثَنَا إِسْرَائِيلُ، عَنْ مَجْزَأَةَ بْنِ زَاهِرٍ الأَسْلَمِيِّ، عَنْ أَبِيهِ ـ وَكَانَ مِمَّنْ شَهِدَ الشَّجَرَةَ ـ قَالَ إِنِّي لأُوقِدُ تَحْتَ الْقِدْرِ بِلُحُومِ الْحُمُرِ إِذْ نَادَى مُنَادِي رَسُولِ اللَّهِ صلى الله عليه وسلم إِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم يَنْهَاكُمْ عَنْ لُحُومِ الْحُمُرِ‏.‏
Reference : Sahih al-Bukhari 4173
In-book reference : Book 64, Hadith 215
USC-MSA web (English) reference : Vol. 5, Book 59, Hadith 491
  (deprecated numbering scheme)