Sahih Bukhari, Chapter: Shortening the Prayers (At-Taqseer)

I saw the Prophet on his Mount praying Nawafil by nodding his head, whatever direction he faced, but Allah's Apostle never did the same in offering the compulsory prayers. Narrated Salim: At night'Abdullah bin 'Umar used to offer the prayer on the back of his animal during the journey and never cared about the direction he faced. Ibn 'Umar said, "Allah's Apostle used to offer the optional prayer on the back of his Mount facing any direction and also used to pray the Witr on it but never offered the compulsory prayer on it."