It is narrated from Nafi' that Sayyiduna Ibn Umar (may Allah be pleased with them both) used to offer (voluntary) prayer on his mount, in whichever direction it carried him, and he would also offer witr on it, and he would say: "the Messenger of Allah (peace and blessings be upon him) also used to do the same."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2213