Sahih al-Bukhari 1216
Narrated `Abdullah:
I used to greet the Prophet (ﷺ) while he was in prayer and he would return my greeting, but when we 
returned (from Ethiopia) I greeted the Prophet (while he was praying) but he did not return the 
greeting, and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious 
matter)."
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا ابْنُ فُضَيْلٍ، عَنِ الأَعْمَشِ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنْتُ أُسَلِّمُ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَىَّ، فَلَمَّا رَجَعْنَا سَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ وَقَالَ ‏
"‏ إِنَّ فِي الصَّلاَةِ شُغْلاً ‏"‏‏.‏