Musannaf Ibn Abi Shayba 3:5916

Yazid ibn Harun narrated to us, he said: Yahya ibn Sa’id informed us, from al-Qasim ibn Muhammad, from Sahl ibn Abi Hathma, regarding the prayer of fear (Salat al-Khawf). He said: The Imam stands facing the Qibla, and with him is a group, while another group faces the enemy. He prays one rak'ah with those who are with him. When he stands, those behind him pray one rak'ah for themselves, prostrate, and perform the taslim. Then they go until they stand in the place of their brothers who are facing the enemy. The others return on their tracks and stand behind the Imam. He then prays another rak'ah with them, then performs the taslim. Then those who were behind them stand, perform one rak'ah for themselves, prostrate, and perform the taslim.