It is narrated from Sayyiduna Jabir bin Abdullah al-Ansari (may Allah be pleased with him) that he said: I saw the Messenger of Allah (peace and blessings be upon him) during the expedition of Anmar offering voluntary prayer on his mount while facing the east.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2203