Sahih Bukhari Volume 006, Book 060, Hadith Number 059.
Narrated By Nafi' : 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 Rakat, 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 Rakat. 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 Apostle (See Hadith No. 451, Vol 5 to know exactly "The Fear Prayer.")