Musannaf Ibn Abi Shayba 3:5918

Ghundar narrated to us, from Shu'bah, from Mughirah, from al-Sha'bi, from Masruq, that he said regarding the prayer of fear: The Imam stands and they form two rows behind him. Then the Imam bows, and those who are next to him bow, and he prostrates with those who are next to him. When he stands, those who were next to him move back, and the others come and stand in their place. He bows with them and prostrates with them, while the others are standing. Then they stand and complete one rak'ah each. Thus, the Imam has two rak'ahs in congregation, and the people have one rak'ah each in congregation, and they complete the second rak'ah.