Sahih Muslim Book - 5, Hadith - 1201

حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى 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 فِى الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِى سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا فَقُلْنَا يَا رَسُولَ اللَّهِ كُنَّا نُسَلِّمُ عَلَيْكَ فِى الصَّلاَةِ فَتَرُدُّ عَلَيْنَا. فَقَالَ « إِنَّ فِى الصَّلاَةِ شُغُلاً ».

It was narrated that 'Abdullah said: "We used to greet the Messenger of Allah (s.a.w) when he was offering Salat, and he would return the greeting. When we came back from being with An-Najashi (after the first emigration to Ethiopia), we greeted him and he did not respond. We said: 'O Messenger of Allah, we used to greet you when you were in Salat and you would return the greeting.' He said: 'Indeed during the Salat one is engaged."'