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.
Sahih al-Bukhari Book 65, Hadith 57
← Previous Hadith On the day of Al-Khandaq (the battle of the Trench). the Prophet (ﷺ) said, "They (i.e. pagans prevented us from offering the middle (the Best) Prayer till the sun had set. May Allah fill their graves, their houses (or their bodies) with fire."

Sahih al-Bukhari Book 65, Hadith 56 Next Hadith → Whenever `Abdullah bin `Umar was asked about Salat-al-Khauf (i.e. prayer of fear) he said, "The Imam comes forward with a group of people and leads them in a one rak`a prayer while another group from them who has not prayed yet, stay between the praying group and the enemy. When those who are with the Imam have finished their one rak`a, they retreat and take the positions of those who have not prayed but they will not finish their prayers with Taslim. Those who have not prayed, come forward to offer a rak`a with the Imam (while the first group covers them from the enemy). Then the Imam, having offered two rak`at, finishes his prayer. Then each member of the two groups offer the second rak`a alone after the Imam has finished his prayer. Thus each one of the two groups will have offered two rak`at. But if the fear is too great, they can pray standing on their feet or riding on their mounts, facing the Qibla or not." Nafi` added: I do not think that `Abdullah bin `Umar narrated this except from Allah's Messenger (ﷺ) (See Hadith No. 451, Vol 5 to know exactly "The Fear Prayer.")

Sahih al-Bukhari Book 65, Hadith 58