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 used to be in prayers, and he used to reply to our greetings. But when we came back from Najashi (the King of Ethiopia) we greeted him (while he was praying) and he did not reply to us. We said, “O Allah’s Apostle! We used to greet you in the past and you used to reply to us.” He said, “Verily The Mind is occupied and busy with more important matter during the prayer." (So one cannot return One’s greetings.)
Bukhari Vol. 5 : No. 215