Uthman bin al-Mughira al-Thqfy عثمان بن المغيرة الثقفي مولاهم أبو المغي · The Book on Salat (Prayer)

"We used to talk behind Allah's Messenger (S) during the Salat, a man among us would talk to his companions next to him until (the following) was revealed: And stand before Allah with obedience. (2:238) So we were ordered to be silent and prohibited from talking."