Actions while Praying

Bukhari :: Book 2 :: Volume 22 :: Hadith 307

Narrated'Abdullah:

I used to greet the Prophet 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)."