Sahih Muslim
Sahih Muslim 700 d
It is permissible to offer voluntary prayers atop one’s mount when travelling, no matter what direction it is facing
Arabic hadith text
وَحَدَّثَنَاهُ أَبُو كُرَيْبٍ، أَخْبَرَنَا ابْنُ الْمُبَارَكِ، وَابْنُ أَبِي زَائِدَةَ، ح وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي كُلُّهُمْ، عَنْ عَبْدِ الْمَلِكِ، بِهَذَا الإِسْنَادِ نَحْوَهُ . وَفِي حَدِيثِ ابْنِ مُبَارَكٍ وَابْنِ أَبِي زَائِدَةَ ثُمَّ تَلاَ ابْنُ عُمَرَ { فَأَيْنَمَا تُوَلُّوا فَثَمَّ وَجْهُ اللَّهِ} وَقَالَ فِي هَذَا نَزَلَتْ
Translation
This hadith has been narrated by another chain of transmitters and in the one narrated by Ibn Mubarak and Ibn Abu Za'ida (these words are narrated). Ibn 'Umar then recited
" Whether you turn thither is Allah's face," and it was revealed in this context.