Sahih Muslim , Book 4, Hadith 1497 (USC-MSA) ⓘ
Ibn 'Umar reported that the Messenger of Allah used to say Nafl prayer on (the back of) his camel in whatever direction it took him.
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 .
also referenced under: Book 6, Hadith 41 (In-Book) & Hadith 700 (Reference)