From Sahih Muslim (Hadith 1205):
Jabir reported:The Messenger of Allah (ﷺ) sent me on an errand. I (having done the business assigned to me came back and) joined him as he was going (on a ride). Qutaiba said that he was saying prayer while he rode. I greeted him. He gestured to me. When he completed the prayer. he called me and said: You greeted me just now while I was engaged in prayer. (Qutaiba said): His (Prophet's face) was towards the east, as he was praying
Reference: Muslim 1205