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  

We were in the company of the Messenger of Allah ﷺ, and he sent me on an errand, and when I came back (I saw him) saying prayer on his ride and his face was not turned towards Qibla. I greeted him but he did not respond to me. As he completed the prayer, he said: Nothing prevented me from responding to your greeting but the fact that I was praying.  

كُنَّا مَعَ النَّبِيِّ ﷺ فَبَعَثَنِي فِي حَاجَةٍ فَرَجَعْتُ وَهُوَ يُصَلِّي عَلَى رَاحِلَتِهِ وَوَجْهُهُ عَلَى غَيْرِ الْقِبْلَةِ فَسَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ فَلَمَّا انْصَرَفَ قَالَ إِنَّهُ لَمْ يَمْنَعْنِي أَنْ أَرُدَّ عَلَيْكَ إِلاَّ أَنِّي كُنْتُ أُصَلِّي