سنن النسائي

It was narrated that Zaid bin Arqam said: "We used to speak to each other during the prayer, saying whatever was necessary, at the time of the Messenger of Allah (ﷺ), until this verse was revealed: Guard strictly (five obligatory) As-Salawat (the prayers) especially the middle Salah (i.e. the best prayer- 'Asr). And stand before Allah with obedience (and do not speak to others during the Salah (prayers)), so we were commanded to be silent."