Language:
كتاب العمل فى الصلاة
21  
Actions while Praying
(2)
Chapter: What speech is prohibited during As-Salat
(2)
باب مَا يُنْهَى عَنْهُ مِنَ الْكَلاَمِ فِي الصَّلاَةِ
Sahih al-Bukhari 1199

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)."

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 1199
In-book reference : Book 21, Hadith 2
USC-MSA web (English) reference : Vol. 2, Book 22, Hadith 290
  (deprecated numbering scheme)