Sahih al-Bukhari 1216
Sahih al-Bukhari · Book 21: Actions while Praying · One should not return greetings during the Salat
Narrated `Abdullah:
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 " .
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)."