Abu Dawud Book 2, Hadith Number 1407.

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 Ibn ‘Umar : The Apostle of Allah (PBUH) would recite to us a surah (according to the vesion of Ibn Mumair) outside the prayer (the agreed version goes), then we would prostrate along with him, and none of us could find a place for his forehead.