Sunan Abi Dawud, Hadith 1220 ⓘ
Narrated Ibn 'Umar: While travelling the Messenger of Allah would pray voluntary prayer on his riding beast in whatever direction it turned; and he would observe witr prayer, but he did not offer the obligatory prayers upon it.
حَدَّثَنَا أَحْمَدُ بْنُ صَالِحٍ، حَدَّثَنَا ابْنُ وَهْبٍ، أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، عَنْ سَالِمٍ، عَنْ أَبِيهِ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُسَبِّحُ عَلَى الرَّاحِلَةِ أَىَّ وَجْهٍ تَوَجَّهَ وَيُوتِرُ عَلَيْهَا غَيْرَ أَنَّهُ لاَ يُصَلِّي الْمَكْتُوبَةَ عَلَيْهَا .
also referenced under: Hadith 1224 (DarusSalam) & Book 4, Hadith 1220 (USC-MSA)