Yahya related to me from Malik from Nafi that Abdullah ibn Umar, when asked about the fear prayer said, "The imam and a group of people go forward and the imam prays a raka with them, while another group, who have not yet prayed, position themselves between him and the enemy. When those who are with him have prayed a raka they draw back to where those who have not prayed are, and do not say the taslim. Then those who have not prayed come forward and pray a raka with him. Then the imam leaves, as he has now prayed two rakas. Everyone else in the two groups stands and prays a raka by himself after the imam has left. In this way each of the two groups will have prayed two rakas. If the fear is greater than that, then the men pray standing on their feet or mounted, either facing the qibla or otherwise."
