Sahih al-Bukhari Hadith 215 – Volume 5, Book 58

English

Volume 5, Book 58, Number 215:
Narrated 'Abdullah:
We used to greet the Prophet 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.)