"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 فَيَرُدُّ عَلَيْنَا السَّلَامَ حَتَّى قَدِمْنَا مِنْ أَرْضِ الْحَبَشَةِ فَسَلَّمْتُ عَلَيْهِ فَلَمْ يُرِدَّ عَلَيَّ فَأَخَذَنِي مَا قَرُبَ وَمَا بَعُدَ فَجَلَسْتُ حَتَّى إِذَا قَضَى الصَّلَاةِ قَالَ «إِنَّ اللهَ يُحْدِثُ مِنْ أَمْرِهِ مَا يَشَاءُ وَقَدْ أَحْدَثَ مِنْ أَمْرِهِ أَنْ لَا يُتَكَلَّمَ فِي الصَّلَاةِ»