Sahih Bukhari Volume 2, Book 18, Hadith 1099

وَقَالَ اللَّيْثُ حَدَّثَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ قَالَ سَالِمٌ كَانَ عَبْدُ اللَّهِ يُصَلِّي عَلَى دَابَّتِهِ مِنَ اللَّيْلِ وَهْوَ مُسَافِرٌ، مَا يُبَالِي حَيْثُ مَا كَانَ وَجْهُهُ‏.‏ قَالَ ابْنُ عُمَرَ وَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُسَبِّحُ عَلَى الرَّاحِلَةِ قِبَلَ أَىِّ وَجْهٍ تَوَجَّهَ، وَيُوتِرُ عَلَيْهَا، غَيْرَ أَنَّهُ لاَ يُصَلِّي عَلَيْهَا الْمَكْتُوبَةَ‏

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."