It was narrated that Jabir said: "The Messenger of Allah (ﷺ) sent me on an errand then I came back to him while he was praying. I greeted him with the salam and he gestured to me. When he finished he called me and said: “You greeted me with Salam just now and I was praying.' And he was facing toward the east that day”"
SahihSahihSahih MuslimBook 13 · Hadith 11