حَدَّثَنَا إِسْمَاعِيلُ حَدَّثَنَا أَيُّوبُ عَنْ سَعِيدِ بْنِ جُبَيْرٍ أَنَّ ابْنَ عُمَرَ كَانَ يُصَلِّي عَلَى رَاحِلَتِهِ تَطَوُّعًا فَإِذَا أَرَادَ أَنْ يَسْجُدَ نَزَلَ فَأَوْتَرَ عَلَى الْأَرْضِ
مسند امام احمد بن حنبل · Hadith 4476
It was narrated from Sa'eed ibn Jubayr that Ibn 'Umar used to offer voluntary prayers on his mount, then when he wanted to pray Witr, he would dismount and pray Witr on the ground.
Translated by Dr. Muhammad Muhsin Khan · Reviewed by Islamic University of Madinah · Published March 2020