Sahih Bukhari Volume 002, Book 022, Hadith Number 308.
Narated By 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.