Abu Dawud Book 2, Hadith Number 1223.
Chapter : To offer the supererogatory prayer and witr prayer on a riding beast.
Narated By Jabir ibn Abdullah : The Apostle of Allah (PBUH) sent me on some business, and when I came to him he was praying on (the back of) his riding beast (moving) towards the east and making the prostration lower than the bowing.