(15) باب لاَ يَرُدُّ السَّلاَمَ فِي الصَّلاَةِ
Narrated Jabir bin `Abdullah:
Allah's Messenger (ﷺ) sent me for some job and when I had finished it I returned and came to the Prophet (ﷺ) and greeted him but he did not return my greeting. So I felt so sorry that only Allah knows it and I said to myself,, 'Perhaps Allah's Messenger (ﷺ) is angry because I did not come quickly, then again I greeted him but he did not reply. I felt even more sorry than I did the first time. Again I greeted him and he returned the greeting and said, "The thing which prevented me from returning the greeting was that I was praying." And at that time he was on his Rahila and his face was not towards the Qibla.
Reference: Sahih al-Bukhari 1217
In-book reference: Book 21, Hadith 21
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 22, Hadith 308