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)."  

كُنْتُ أُسَلِّمُ عَلَى النَّبِيِّ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَىَّ فَلَمَّا رَجَعْنَا سَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ وَقَالَ إِنَّ فِي الصَّلاَةِ شُغْلاً  

Messenger of Allah 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 Messenger of Allah 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.  

بَعَثَنِي رَسُولُ اللَّهِ ﷺ فِي حَاجَةٍ لَهُ فَانْطَلَقْتُ ثُمَّ رَجَعْتُ وَقَدْ قَضَيْتُهَا فَأَتَيْتُ النَّبِيَّ ﷺ فَسَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ فَوَقَعَ فِي قَلْبِي مَا اللَّهُ أَعْلَمُ بِهِ فَقُلْتُ فِي نَفْسِي لَعَلَّ رَسُولَ اللَّهِ ﷺ وَجَدَ عَلَىَّ أَنِّي أَبْطَأْتُ عَلَيْهِ ثُمَّ سَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ فَوَقَعَ فِي قَلْبِي أَشَدُّ مِنَ الْمَرَّةِ الأُولَى ثُمَّ سَلَّمْتُ عَلَيْهِ فَرَدَّ عَلَىَّ فَقَالَ إِنَّمَا مَنَعَنِي أَنْ أَرُدَّ عَلَيْكَ أَنِّي كُنْتُ أُصَلِّي وَكَانَ عَلَى رَاحِلَتِهِ مُتَوَجِّهًا إِلَى غَيْرِ الْقِبْلَةِ