I used to greet the Prophet (ﷺ) while he was in prayer and he would return my greeting, but when we returned (from Ethiopia) I greeted the Prophet (while he was praying) but he did not return the greeting, and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)."
Sahih al-Bukhari Book 21, Hadith 20
← Previous Hadith The people used to offer the prayer with the Prophet (ﷺ) with their waist-sheets tied round their necks because of the shortness of the sheets and the women were ordered not to lift their heads till the men had sat straight.

Sahih al-Bukhari Book 21, Hadith 19 Next Hadith → Allah's Messenger (ﷺ) sent me for some job and when I had finished it I returned and came to the Prophet (ﷺ) and greeted him but he did not return my greeting. So I felt so sorry that only Allah knows it and I said to myself,, 'Perhaps Allah's Messenger (ﷺ) is angry because I did not come quickly, then again I greeted him but he did not reply. I felt even more sorry than I did the first time. Again I greeted him and he returned the greeting and said, "The thing which prevented me from returning the greeting was that I was praying." And at that time he was on his Rahila and his face was not towards the Qibla.

Sahih al-Bukhari Book 21, Hadith 21