(4) باب جَوَازِ صَلاَةِ النَّافِلَةِ عَلَى الدَّابَّةِ فِي السَّفَرِ حَيْثُ تَوَجَّهَتْ
'Abdullah b. 'Amir b. Rabi'a has reported on the authority of his father that he had seen the Messenger of Allah (ﷺ) observing Nafl player at night on a journey on the back of his ride in whichever direction it turned its face.
Reference: Sahih Muslim 701
In-book reference: Book 6, Hadith 50
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 1506