Musnad Abu Ya'la al-Mawsili 108:20

Ibrahim narrated to us, Hammad narrated to us, from Abu al-Zubayr, from Jabir, that they slaughtered horses, mules, and domestic donkeys on the day of Khaybar. The Messenger of Allah (may Allah bless him and grant him peace) forbade mules and donkeys, but he did not forbid horses.