Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) said concerning the fear prayer: "The Imam should lead one group in prayer, and they should perform one prostration, and there should be another group between them and the enemy (guarding them). Then those who did the prostration with their leader should move away, and take the place of those who have not yet prayed. Then those who have not yet prayed should come forward and perform one prostration with their leader. Then their leader should move away, and his prayer will be complete. Then each group should perform one prostration by itself. If the fear is too great, then (they should pray) on foot or riding.'" He said: What is meant by prostration here is a Rakah. Sahih