Prayer (Kitab Al-Salat): Detailed Rules of Law about the Prayer during Journey
Dawud :: Book 4 : Hadith 1223
Narrated Jabir ibn Abdullah:
The Apostle of Allah (peace_be_upon_him) 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.