Nafi' (may Allah have mercy on him) said that Abdullah bin Umar (may Allah be pleased with them both) used to pray voluntary prayer on his camel in whichever direction it faced, and he told them that he (peace and blessings be upon him) used to do so, and Abdullah bin Umar (may Allah be pleased with them both) used to pray witr on his camel.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4595