Sahih al-Bukhari 4534
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.
حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَحْيَى، عَنْ إِسْمَاعِيلَ بْنِ أَبِي خَالِدٍ، عَنِ الْحَارِثِ بْنِ شُبَيْلٍ، عَنْ أَبِي عَمْرٍو الشَّيْبَانِيِّ، عَنْ زَيْدِ بْنِ أَرْقَمَ، قَالَ كُنَّا نَتَكَلَّمُ فِي الصَّلاَةِ يُكَلِّمُ أَحَدُنَا أَخَاهُ فِي حَاجَتِهِ حَتَّى نَزَلَتْ هَذِهِ الآيَةُ ‏{‏حَافِظُوا عَلَى الصَّلَوَاتِ وَالصَّلاَةِ الْوُسْطَى وَقُومُوا لِلَّهِ قَانِتِينَ‏}‏ فَأُمِرْنَا بِالسُّكُوتِ‏.‏