Sahih Bukhari
Actions while Praying
Volume: 002 / Book Number: 022 / Hadith Number: 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)."