كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(4)
Chapter: It is permissible to offer voluntary prayers atop one’s mount when travelling, no matter what direction it is facing
(4)
باب جَوَازِ صَلاَةِ النَّافِلَةِ عَلَى الدَّابَّةِ فِي السَّفَرِ حَيْثُ تَوَجَّهَتْ ‏‏
Sahih Muslim 700 d

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.
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 ‏.‏
Reference : Sahih Muslim 700d
In-book reference : Book 6, Hadith 44
USC-MSA web (English) reference : Book 4, Hadith 1500
  (deprecated numbering scheme)