[Machine] People used to return greetings to each other in prayer, until Ibn Mas'ud greeted the Prophet ﷺ and he did not respond. So Ibn Mas'ud felt sad, thinking that something might have happened to him. When the Prophet ﷺ finished his prayer, he mentioned that to Ibn Mas'ud, and the Prophet ﷺ said, "Indeed, in prayer, there is occupation, and it is sufficient as occupation."  

كَانَ النَّاسُ يَرُدُّ بَعْضُهُمْ عَلَى بَعْضٍ السَّلَامَ فِي الصَّلَاةِ حَتَّى سَلَّمَ ابْنُ مَسْعُودٍ عَلَى النَّبِيِّ ﷺ فَلَمْ يَرُدَّ عَلَيْهِ فَقَعَدَ حَزِينًا يُخَيَّلُ إِلَيْهِ أَنَّهُ قَدْ نَزَلَ فِيهِ شَيْءٌ فَلَمَّا قَضَى النَّبِيُّ ﷺ صَلَاتَهُ ذَكَرَ ذَلِكَ لَهُ ابْنُ مَسْعُودٍ فَقَالَ النَّبِيُّ ﷺ «إِنَّ فِي الصَّلَاةِ لَشُغْلًا وَكَفَى بِالصَّلَاةِ شُغْلًا»