Musannaf Ibn Abi Shayba 16:93

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from al-Hakam, from Mujahid, that he was asked about the meat of horses. He said: "And the horses, the mules, and the donkeys, that you may ride them." It was as if he disliked their meat.