كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهْوَ يُصَلِّي فَيَرُدُّ عَلَيْنَا، فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا، فَقُلْنَا يَا رَسُولَ اللَّهِ إِنَّا كُنَّا نُسَلِّمُ عَلَيْكَ فَتَرُدُّ عَلَيْنَا قَالَ إِنَّ فِي الصَّلاَةِ شُغْلاً . فَقُلْتُ لإِبْرَاهِيمَ كَيْفَ تَصْنَعُ أَنْتَ قَالَ أَرُدُّ فِي نَفْسِي.
We used to greet the Prophet (ﷺ) while he used to be in prayers, and he used to reply to our greetings. Butwhen we came back from Najashi (the King of Ethiopia) we greeted him (while he was praying) andhe did not reply to us. We said, "O Allah's Messenger (ﷺ)! We used to greet you in the past and you used toreply to us." He said, "Verily The Mind is occupied and busy with more important matter during theprayer." (So one cannot return One's greetings.)