Thuwayr bin Abi Fakhitah narrates from his father that Sayyiduna Ali (may Allah be pleased with him) used to offer witr on his mount. (In the chain through al-Thawri there is also the addition that he would perform (the bowing and prostration) by gestures.)
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2217