Narrated `Abdullah: I used to greet the Prophet 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)."
Hadith Referenceصحيح البخاري / كتاب العمل في الصلاة / 1216
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة
Narrated Jabir bin `Abdullah: Allah's Apostle sent me for some job and when I had finished it I returned and came to the Prophet 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 Apostle 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.
Hadith Referenceصحيح البخاري / كتاب العمل في الصلاة / 1217
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة