From Sahih Bukhari (Hadith 1216):
Narrated `Abdullah: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)
Reference: Bukhari 1216