Musnad Ahmad 5:415

Aswad ibn Amir narrated to us, Isra'il informed us, from Muhammad ibn Ubayd Allah, from his father, from his uncle, who said: Ali was asked: "May a man ride his sacrificial animal (hady)?" He said: "There is no harm in it. The Prophet, peace and blessings of Allah be upon him, used to pass by men who were walking, and he would command them to ride his sacrificial animal—the sacrificial animal of the Prophet, peace and blessings of Allah be upon him." He said: "And do not follow anything better than the Sunnah of your Prophet, peace and blessings of Allah be upon him."