Bismillah Walhamdulillah Was Salaatu Was Salaam ‘ala Rasulillah

Narrated ‘Abdullah (Radi-Allahu ‘anhu):

We used to greet the Prophet (Sallallahu ‘Alaihi Wa Sallam) while he was praying and he used to answer our greetings. When we returned from AnNajashi (the ruler of Ethiopia), we greeted him, but he did not answer us (during the prayer) and (after finishing the prayer) he said, “In the prayer one is occupied (with a more serious matter).”

Bukhari Vol. 2 : No. 290