Abu Dawud Book 2, Hadith Number 1406.

Chapter : If a person listens to a verse in which a prostration occurs and he is riding the beast what should he do.

Narated By Abdullah ibn Umar : In the year of Conquest the Apostle of Allah (PBUH) recited a verse at which a prostration should be made and all the people prostrated themselves. Some were mounted, and some were prostrating themselves on the ground, and those who were mounted prostrated themselves on their hands.