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. (Using translation from Bukhārī 1217)
أَرْسَلَنِي رَسُولُ اللهِ ﷺ فِي حَاجَةٍ فَانْطَلَقْتُ ثُمَّ رَجَعْتُ وَقَدْ قَضَيْتُهَا فَأَتَيْتُ النَّبِيَّ ﷺ فَسَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَيَّ فَوَقَعَ فِي قَلْبِي مَا اللهُ أَعْلَمُ بِهِ فَقُلْتُ فِي نَفْسِي لَعَلَّ رَسُولَ اللهِ ﷺ وَجَدَ عَلَيَّ أَنَّنِي أَبْطَأْتُ عَلَيْهِ ثُمَّ سَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَيَّ فَوَقَعَ فِي نَفْسِي أَشَدُّ مِنَ الْمَرَّةِ الْأُولَى ثُمَّ سَلَّمْتُ عَلَيْهِ فَرَدَّ عَلَيَّ فَقَالَ أَمَا إِنَّهُ لَمْ يَمْنَعْنِي أَنْ أَرُدَّ عَلَيْكَ إِلَّا أَنِّي كُنْتُ أُصَلِّي وَكَانَ عَلَى رَاحِلَتِهِ مُتَوَجِّهًا لِغَيْرِ الْقِبْلَةِ