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.
USC-MSA web (English) reference: Vol. 6, Book 60, Hadith 58
Arabic reference: Book 65, Hadith 4534