Musnad Ahmad 28:167

Hassan narrated to us, Ibn Lahi'ah narrated to us, Huyayy ibn Abdullah narrated to me, that Abu Abd al-Rahman al-Hubuli narrated to him, he said: I heard Abdullah ibn Amr saying: "Surah al-Ma'idah was revealed to the Messenger of Allah, may Allah bless him and grant him peace, while he was riding upon his mount, and it could not bear him, so he descended from it."