Sahih Bukhari

Narrated Zaid bin Arqam:We used to speak while in prayer. One of us used to speak to his brother (while in prayer) about his need, till the Verse was revealed:-- ""Guard strictly the (five obligatory) prayers, especially the middle (the Best) (`Asr) Prayer and stand before Allah with obedience (and not to speak to others during the prayers)."" Then we were ordered not to speak in the prayers