The fear prayer consisted of only two prostration units, like the prayer your guards perform today behind their leaders, except that it was done in turns. A group of them stood while all of them were present with the Messenger of Allah ﷺ, and one group prostrated with him. Then the Messenger of Allah ﷺ stood up, 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 on their own, then sat down. Then the Messenger of Allah ﷺ joined them all together with the closing salutation.
أخبرنا عبيد الله بن سعد بن إبراهيم قال حدثنا عمي قال حدثنا أبي عن ابن إسحق قال حدثني داود بن الحصين عن عكرمة
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
[hasan] Ahmad relates it, 1/265, from the narration of Ibrahim bin Saad, from Ibn Ishaq, with this chain. It is also found in al-Kubra, no. 1923, but it contains a serious defect. It has a hasan supporting narration in Abu Dawud, no. 1242, and elsewhere. Ibn Khuzayma graded it sahih, no. 1363, as did Ibn Hibban, no. 589, and Hakim, 1/336, and Dhahabi.