Sahih al-Bukhari 4534
Sahih al-Bukhari · Book 65: Prophetic Commentary on the Qur'an (Tafseer of the Prophet (pbuh)) · "...And stand before Allah with obedience [and do not speak to others during the Salat (prayers)]” (V.2:238)
Narrated Zaid bin Arqam:
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.
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.