كتاب السهو
Read the English translation first, with the original Arabic directly underneath.
It was narrated that Ibn Mas'ud said:
"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.'"
إِنَّ اللَّهَ عَزَّ وَجَلَّ يُحْدِثُ مِنْ أَمْرِهِ مَا يَشَاءُ وَإِنَّهُ قَدْ أَحْدَثَ مِنْ أَمْرِهِ أَنْ لاَ يُتَكَلَّمَ فِي الصَّلاَةِ