He greeted the Messenger of Allah (ﷺ) with the Salam when he was praying, and he returned the greeting.
Sunan an-Nasa'i Book 13, Hadith 10
← Previous Hadith "Ibn 'Umar said: The Prophet (ﷺ) entered the Masjid of Quba' to pray there, then some men came in and greeted him with Salam. I asked Suhaib, who was with him: 'What did the Messenger of Allah (ﷺ) do when he was greeted?' He said: 'He used to gesture with his hand.'"

Sunan an-Nasa'i Book 13, Hadith 9 Next Hadith → "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."

Sunan an-Nasa'i Book 13, Hadith 11