Abdullah (b. Masu'd) reported: We used to greet the Messenger of Allah ﷺ while he was engaged in prayer and he would respond to our greeting. But when we returned from the Negus we greeted him and he did not respond to us;
so we said: Messenger of Allah. we used to greet you when you were engaged in prayer and you would respond to us. He replied: Prayer demands whole attention.

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، وَزُهَيْرُ بْنُ حَرْبٍ، وَابْنُ، نُمَيْرٍ وَأَبُو سَعِيدٍ الأَشَجُّ - وَأَلْفَاظُهُمْ مُتَقَارِبَةٌ - قَالُوا حَدَّثَنَا ابْنُ فُضَيْلٍ، حَدَّثَنَا الأَعْمَشُ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ، قَالَ كُنَّا نُسَلِّمُ عَلَى رَسُولِ اللَّهِ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِي سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا فَقُلْنَا يَا رَسُولَ اللَّهِ كُنَّا نُسَلِّمُ عَلَيْكَ فِي الصَّلاَةِ فَتَرُدُّ عَلَيْنَا . فَقَالَ " إِنَّ فِي الصَّلاَةِ شُغُلاً " .