Ibn Umar used to perform voluntary prayers on his mount, but when he wanted to perform the Witr prayer, he would dismount and pray it on the ground.
حدثنا إسماعيل حدثنا أيوب
أن ابن عمر كان يصلي على راحلته تطوعا فإذا أراد أن يسجد نزل فأوتر على الأرض
No references are available.
Its isnad is sahih. Ismail is Ibn Ulaya. This is a mawquf report from Ibn Umar concerning his own practice, but he himself narrated that the Prophet ﷺ used to perform witr while riding, as in al-Muntaqa 833, and as will come at 4519, and see 4470.