Musannaf Ibn Abi Shayba 3:6145

Yazid ibn Harun narrated to us, from Ibn 'Awn, who said: I asked al-Qasim ibn Muhammad, "Does a man pray upon his riding beast?" He said, "Yes." I said, "Does he pray in whatever direction it is facing?" He said, "Yes." I said, "Does he make the prostration lower than the bowing?" He said, "Yes."