It wasnarrated that Ibn 'Umar said:
"The Messenger of Allah (ﷺ)saidconcerning the fear prayer: "The Imam should lead one group inprayer, and they should perform one prostration, and there should beanother group between them and the enemy (guarding them). Then thosewho did the prostration with their leader should move away, and takethe place of those who have not yet prayed. Then those who have notyet prayed should come forward and perform one prostration with theirleader. Then their leader should move away, and his prayer will becomplete. Then each group should perform one prostration by itself.Ifthe fear is too great, then (they should pray) on foot orriding.'" Hesaid: What is meant by prostration here is a Rak'ah.
English reference: Vol. 1, Book 5, Hadith 1258
Arabic reference: Book 5, Hadith 1316