Sahih Al Bukhari Hadith Number 1216

Narrated `Abdullah: I used to greet the Prophet (peace be upon him) 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).”

21  Actions while Praying
Hadith number 1216
Volume no 2
Chapter: 15 (One should not return greetings during the Salat)