Sa'id bin Jubayr narrated: Sayyiduna Abdullah bin Umar would offer voluntary prayer on his mount; when he wanted to offer witr, he would dismount from the mount and offer witr on the ground.
Imam Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2987