Sahih al-Bukhari 1093
Sahih al-Bukhari · Book 18: Shortening the Prayers (At-Taqseer) · To offer Nawafil on the back of animals
Narrated `Abdullah bin 'Amir from his father who said:
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ ، قَالَ حَدَّثَنَا عَبْدُ الأَعْلَى ، قَالَ حَدَّثَنَا مَعْمَرٌ ، عَنِ الزُّهْرِيِّ ، عَنْ عَبْدِ اللَّهِ بْنِ عَامِرٍ ، عَنْ أَبِيهِ ، قَالَ رَأَيْتُ النَّبِيَّ صلى الله عليه وسلم يُصَلِّي عَلَى رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ بِهِ.
I saw the Prophet (p.b.u.h) offering the prayer on his mount (Rahila) whatever direction it took.