It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) used to offer voluntary prayer on the back of his mount. He (peace and blessings be upon him) paid no attention to whichever way his face was turned, and he (peace and blessings be upon him) would make gestures with his head.
And Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) also used to do the same.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2209