"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."
Reference: Sunan an-Nasa'i 1189 In-book : Book 13, Hadith 11 English translation : Vol. 2, Book 13, Hadith 1190
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