Narrated `Abdullah: We used to greet the Prophet (ﷺ) while he was praying and he used to answer our greetings. When we returned from An-Najashi (the ruler of Ethiopia), we greeted him, but he did not answer us (during the prayer) and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا ابْنُ فُضَيْلٍ، حَدَّثَنَا الأَعْمَشُ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا، فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا وَقَالَ " إِنَّ فِي الصَّلاَةِ شُغْلاً ".
sunnah.com