"We used to greet the Prophet ﷺ with salam and he would return our salam, until we came back from the land of Ethiopia. I greeted him with salam and he did not return my greeting,a nd I started to wonder why. So I sat down; and when he finished praying, he said: 'Allah (SWT) decrees what He wills, and He has decreed what we should not speak during the prayer.'" (Using translation from Nasāʾī 1221)
كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ ﷺ إِذْ كُنَّا بِمَكَّةَ قَبْلَ أَنْ نَأْتِيَ أَرْضَ الْحَبَشَةِ فَلَمَّا قَدِمْنَا مِنْ أَرْضِ الْحَبَشَةِ أَتَيْنَاهُ فَسَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ فَأَخَذَنِي مَا قَرُبَ وَمَا بَعُدَ حَتَّى قَضَوْا الصَّلَاةَ فَسَأَلْتُهُ فَقَالَ إِنَّ اللهَ ﷻ يُحْدِثُ فِي أَمْرِهِ مَايَشَاءُ وَإِنَّهُ قَدْ أُحْدِثَ مِنْ أَمْرِهِ أَنْ لَا نَتَكَلَّمَ فِي الصَّلَاةِ