The prayer of fear was nothing but two prostrations, just like the prayer your guards perform today behind these imams of yours, except that it was done in turns. A group of them stood while all of them were together with the Messenger of Allah ﷺ, and another group prostrated with him. Then the Messenger of Allah ﷺ stood and they all stood with him. Then he bowed and they all bowed with him. Then he prostrated, and those who had been standing the first time prostrated with him. When the Messenger of Allah ﷺ sat down along with those who had prostrated with him at the end of their prayer, those who had been standing prostrated by themselves. Then they sat, and the Messenger of Allah ﷺ concluded the prayer for all of them with the salutation.
ما كانت صلاة الخوف إلا سجدتين كصلاة أحراسكم هؤلاء اليوم خلف أئمتكم هؤلاء إلا أنها كانت عقبا، قامت طائفة منهم وهم جميعا مع رسول الله ﷺ، وسجدت معه طائفة منهم، ثم قام رسول الله ﷺ وقاموا معه جميعا، ثم ركع وركعوا معه جميعا، ثم سجد فسجد معه الذين كانوا قياما أول مرة، فلما جلس رسول الله ﷺ والذين سجدوا معه في آخر صلاتهم سجد الذين كانوا قياما لأنفسهم، ثم جلسوا فجمعهم رسول الله ﷺ بالتسليم.
Nasai (1535) narrated it from Ubaydullah bin Saad bin Ibrahim, who said, "My uncle told me," who said, "My father told us," from Ibn Ishaq, who said, "Dawud bin Husayn told me," from Ikrima, from Ibn Abbas, and he related it.Ubaydullah's uncle is Yaqub bin Ibrahim bin Saad, and from him Ahmad (2382) and Bayhaqi (3/258) narrated it. Its isnad is hasan on account of Muhammad bin Ishaq, for he is truthful (sadooq) when he states explicitly that he was told directly.His statement that it was "in succession" means that one group would pray after another, taking turns just as raiders take turns.