Abu Dawud Book 2, Hadith Number 1221.

Chapter : To offer the supererogatory prayer and witr prayer on a riding beast.

Narated By Anas ibn Malik : When the Apostle of Allah (PBUH) was on a journey and wished to say voluntary prayer, he made his she-camel face the qiblah and uttered the takbir (Allah is most great), then prayed in whatever direction his mount made his face.