(15) باب لاَ يَرُدُّ السَّلاَمَ فِي الصَّلاَةِ
Narrated `Abdullah:
I used to greet the Prophet (ﷺ) while he was in prayer and he would return my greeting, but when we returned (from Ethiopia) I greeted the Prophet (while he was praying) but he did not return the greeting, and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)."
Reference: Sahih al-Bukhari 1216
In-book reference: Book 21, Hadith 20
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 22, Hadith 307