Sahih al-Bukhari 3875
Narrated `Abdullah:
We used to greet the Prophet (ﷺ) 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 Messenger (ﷺ)! 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.)
حَدَّثَنَا يَحْيَى بْنُ حَمَّادٍ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ سُلَيْمَانَ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ 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.‏