عَنْ سَعِيدِ بْنِ جُبَيْرٍ أَنَّ ابْنَ عُمَرَ رَضِيَ اللَّهُ عَنْهُ كَانَ يُصَلِّي عَلَى رَاحِلَتِهِ تَطَوُّعًا، فَإِذَا أَرَادَ أَنْ يُوْتِرَ نَزَلَ فَأَوْتَرَ عَلَى الْأَرْضِ
Saeed bin Jubair says that Sayyiduna Abdullah bin Umar (may Allah be pleased with him) used to perform voluntary prayers on his mount, but when he intended to perform the Witr prayer, he would dismount and perform Witr on the ground.