(65) Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh))
كتاب التفسير
(43) "...And stand before Allah with obedience [and do not speak to others during the Salat (prayers)]” (V.2:238)
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.‏
Reference : Sahih al-Bukhari 4534
In-book reference : Book 65, Hadith 57
USC-MSA web (English) reference : Vol. 6, Book 60, Hadith 58
  (deprecated numbering scheme)