Language:
كتاب مناقب الأنصار
63  
Merits of the Helpers in Madinah (Ansaar)
(37)
Chapter: The emigration to Ethiopia
(37)
باب هِجْرَةِ الْحَبَشَةِ
Sahih al-Bukhari 3875

Narrated `Abdullah:

We used to greet the Prophet (ﷺ) while he used to be in prayers, and he used to reply to our greetings. But when we came back from Najashi (the King of Ethiopia) we greeted him (while he was praying) and he did not reply to us. We said, "O Allah's Messenger (ﷺ)! We used to greet you in the past and you used to reply to us." He said, "Verily The Mind is occupied and busy with more important matter during the prayer." (So one cannot return One's greetings.)

حَدَّثَنَا يَحْيَى بْنُ حَمَّادٍ، حَدَّثَنَا أَبُو عَوَانَةَ، عَنْ سُلَيْمَانَ، عَنْ إِبْرَاهِيمَ، عَنْ عَلْقَمَةَ، عَنْ عَبْدِ اللَّهِ ـ رضى الله عنه ـ قَالَ كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ صلى الله عليه وسلم وَهْوَ يُصَلِّي فَيَرُدُّ عَلَيْنَا، فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا، فَقُلْنَا يَا رَسُولَ اللَّهِ إِنَّا كُنَّا نُسَلِّمُ عَلَيْكَ فَتَرُدُّ عَلَيْنَا قَالَ ‏ "‏ إِنَّ فِي الصَّلاَةِ شُغْلاً ‏"‏‏.‏ فَقُلْتُ لإِبْرَاهِيمَ كَيْفَ تَصْنَعُ أَنْتَ قَالَ أَرُدُّ فِي نَفْسِي‏.‏
Reference : Sahih al-Bukhari 3875
In-book reference : Book 63, Hadith 100
USC-MSA web (English) reference : Vol. 5, Book 58, Hadith 215
  (deprecated numbering scheme)