Musnad Ahmad 26:14

Muhammad ibn Fudayl narrated to us, from Khasif, Abu Ubaydah narrated to us, from Abdullah, who said: The Messenger of Allah (ﷺ) led us in the prayer of fear. They stood in two rows: one row stood behind the Prophet (ﷺ), and one row stood facing the enemy. The Messenger of Allah (ﷺ) prayed one rak'ah with the row that was next to him. Then they stood up and went, and they stood in the place of the others, facing the enemy. The others came and stood in their place, and the Messenger of Allah (ﷺ) prayed one rak'ah with them, then he performed the taslim. Then they 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. The others returned to their place and prayed one rak'ah for themselves, then they performed the taslim.