‘Abd Allah(b. Mas’ud) said:
We used to salute the Messenger of Allahﷺ while he was engaged in prayer and he would respond to our salutation, but when we returned from the Negas, we saluted him and he did not respond to us. He said : Prayer demands one’s whole attention.

حَدَّثَنَا مُحَمَّدُ بْنُ عَبْدِ اللَّهِ بْنِ نُمَيْرٍ، حَدَّثَنَا ابْنُ فُضَيْلٍ، عَنِ الأَعْمَشِ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنَّا نُسَلِّمُ عَلَى رَسُولِ اللَّهِ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا وَقَالَ " إِنَّ فِي الصَّلاَةِ لَشُغْلاً " .