We used to greet the Prophet ﷺ while he was praying and he used to answer our greetings. When we returned from An-Najashi (the ruler of Ethiopia), we greeted him, but he did not answer us (during the prayer) and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)."  

كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا وَقَالَ إِنَّ فِي الصَّلاَةِ شُغْلاً  

We used to speak while in prayer. One of us used to speak to his brother (while in prayer) about his need, till the Verse was revealed:-- "Guard strictly the (five obligatory) prayers, especially the middle (the Best) (ʿAsr) Prayer and stand before Allah with obedience (and not to speak to others during the prayers)." Then we were ordered not to speak in the prayers.  

كُنَّا نَتَكَلَّمُ فِي الصَّلاَةِ يُكَلِّمُ أَحَدُنَا أَخَاهُ فِي حَاجَتِهِ حَتَّى نَزَلَتْ هَذِهِ الآيَةُ {حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلاَةِ الْوُسْطَى وَقُومُوا لِلَّهِ قَانِتِينَ} فَأُمِرْنَا بِالسُّكُوتِ  

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