question #1 Sahih al-Bukhari 1216, Book 21, Hadith 20 : What is the significance of maintaining focus and concentration during prayer in Islam?

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).”