Sahih Bukhari
Prophetic Commentary on the Quran (Tafseer of the Prophet (PBUH))
Volume: 006 / Book Number: 060 / Hadith Number: 58
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.