Musannaf Ibn Abi Shayba 3:5898

Muhammad ibn Fudayl narrated to us, from Khusayf, from Abu Ubaydah, who said: The Messenger of Allah (may Allah bless him and grant him peace) led us in the prayer of fear. They stood in two rows: a row behind the Prophet (may Allah bless him and grant him peace) and a row facing the enemy. The Messenger of Allah (may Allah bless him and grant him peace) prayed one rak'ah with them. Then the others came and stood in their place, while those [first ones] faced the enemy. The Messenger of Allah (may Allah bless him and grant him peace) prayed one rak'ah with them, then he performed the taslim. Then those [second ones] stood up and prayed one rak'ah for themselves, then they performed the taslim. Then they went and stood in the place of the others, facing the enemy. Then those [first ones] returned to their place and prayed one rak'ah for themselves, then they performed the taslim.