Sunan an-Nasai 15228
In-book no 492أَخْبَرَنَا عِيسَى بْنُ حَمَّادٍ، زُغْبَةُ وَأَحْمَدُ بْنُ عَمْرِو بْنِ السَّرْحِ وَالْحَارِثُ بْنُ مِسْكِينٍ قِرَاءَةً عَلَيْهِ وَأَنَا أَسْمَعُ، - وَاللَّفْظُ لَهُ - عَنِ ابْنِ وَهْبٍ، عَنْ يُونُسَ، عَنِ ابْنِ شِهَابٍ، عَنْ سَالِمٍ، عَنْ أَبِيهِ، قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يُسَبِّحُ عَلَى الرَّاحِلَةِ قِبَلَ أَىِّ وَجْهٍ تَتَوَجَّهُ وَيُوتِرُ عَلَيْهَا غَيْرَ أَنَّهُ لاَ يُصَلِّي عَلَيْهَا الْمَكْتُوبَةَ .
It was narrated from Salim that his father said: "The Messenger of Allah (ﷺ) used to pray voluntary prayers (Usabbih) while riding his mount, facing whatever direction it was facing, and he would pray Witr likewise, but he would not offer the prescribed prayers on it."