Sahih Muslim Book: 6, Hadith: 1229

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 صلى الله عليه وسلم 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 ‏”‏ ‏.‏ Abdullah (b. Masu’d) reported:We used to greet the Messenger of Allah (may peace be upon him) 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: ...