حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا عُبَيْدُ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يُصَلِّي سُبْحَتَهُ حَيْثُمَا تَوَجَّهَتْ بِهِ نَاقَتُهُ ‏.‏

Ibn 'Umar reported that the Messenger of Allah (PBUH) used to say Nafl prayer on (the back of) his camel in whatever direction it took him. Translation Not Available

Book Ref: Sahih Muslim Book 6 Hadith 1497
Web Ref:  Sahih Muslim Book 4 Hadith 700 a