Whenever ʿAbdullah bin ʿUmar was asked about Salat-al-Khauf (i.e. prayer of fear) he said, "The Imam comes forward with a group of people and leads them in a one rakʿa prayer while another group from them who has not prayed yet, stay between the praying group and the enemy. When those who are with the Imam have finished their one rakʿa, they retreat and take the positions of those who have not prayed but they will not finish their prayers with Taslim. Those who have not prayed, come forward to offer a rakʿa with the Imam (while the first group covers them from the enemy). Then the Imam, having offered two rakʿat, finishes his prayer. Then each member of the two groups offer the second rakʿa alone after the Imam has finished his prayer. Thus each one of the two groups will have offered two rakʿat. But if the fear is too great, they can pray standing on their feet or riding on their mounts, facing the Qibla or not." Nafiʿ added: I do not think that ʿAbdullah bin ʿUmar narrated this except from Messenger of Allah ﷺ (See Hadith No. 451, Vol 5 to know exactly "The Fear Prayer.")  

يَتَقَدَّمُ الإِمَامُ وَطَائِفَةٌ مِنَ النَّاسِ فَيُصَلِّي بِهِمِ الإِمَامُ رَكْعَةً وَتَكُونُ طَائِفَةٌ مِنْهُمْ بَيْنَهُمْ وَبَيْنَ الْعَدُوِّ لَمْ يُصَلُّوا فَإِذَا صَلَّوُا الَّذِينَ مَعَهُ رَكْعَةً اسْتَأْخَرُوا مَكَانَ الَّذِينَ لَمْ يُصَلُّوا وَلاَ يُسَلِّمُونَ وَيَتَقَدَّمُ الَّذِينَ لَمْ يُصَلُّوا فَيُصَلُّونَ مَعَهُ رَكْعَةً ثُمَّ يَنْصَرِفُ الإِمَامُ وَقَدْ صَلَّى رَكْعَتَيْنِ فَيَقُومُ كُلُّ وَاحِدٍ مِنَ الطَّائِفَتَيْنِ فَيُصَلُّونَ لأَنْفُسِهِمْ رَكْعَةً بَعْدَ أَنْ يَنْصَرِفَ الإِمَامُ فَيَكُونُ كُلُّ وَاحِدٍ مِنَ الطَّائِفَتَيْنِ قَدْ صَلَّى رَكْعَتَيْنِ فَإِنْ كَانَ خَوْفٌ هُوَ أَشَدَّ مِنْ ذَلِكَ صَلَّوْا رِجَالاً قِيَامًا عَلَى أَقْدَامِهِمْ أَوْ رُكْبَانًا مُسْتَقْبِلِي الْقِبْلَةِ أَوْ غَيْرَ مُسْتَقْبِلِيهَا قَالَ مَالِكٌ قَالَ نَافِعٌ لاَ أُرَى عَبْدَ اللَّهِ بْنَ عُمَرَ ذَكَرَ ذَلِكَ إِلاَّ عَنْ رَسُولِ اللَّهِ ﷺ  

“The Messenger of Allah ﷺ 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 Rak’ah.  

قَالَ رَسُولُ اللَّهِ ﷺ فِي صَلاَةِ الْخَوْفِ أَنْ يَكُونَ الإِمَامُ يُصَلِّي بِطَائِفَةٍ مَعَهُ فَيَسْجُدُونَ سَجْدَةً وَاحِدَةً وَتَكُونُ طَائِفَةٌ مِنْهُمْ بَيْنَهُمْ وَبَيْنَ الْعَدُوِّ ثُمَّ يَنْصَرِفُ الَّذِينَ سَجَدُوا السَّجْدَةَ مَعَ أَمِيرِهِمْ ثُمَّ يَكُونُونَ مَكَانَ الَّذِينَ لَمْ يُصَلُّوا وَيَتَقَدَّمُ الَّذِينَ لَمْ يُصَلُّوا فَيُصَلُّوا مَعَ أَمِيرِهِمْ سَجْدَةً وَاحِدَةً ثُمَّ يَنْصَرِفُ أَمِيرُهُمْ وَقَدْ صَلَّى صَلاَتَهُ وَيُصَلِّي كُلُّ وَاحِدَةٍ مِنَ الطَّائِفَتَيْنِ بِصَلاَتِهِ سَجْدَةً لِنَفْسِهِ فَإِنْ كَانَ خَوْفٌ أَشَدَّ مِنْ ذَلِكَ فَرِجَالاً أَوْ رُكْبَانًا قَالَ يَعْنِي بِالسَّجْدَةِ الرَّكْعَةَ  

[Machine] The Prophet Muhammad ﷺ said, "The prayer of fear is performed by the Imam and a group of people with him. They all prostrate together in one prostration. There is another group between them and the enemy. Then those who prostrated with their leader leave, taking the place of those who did not perform the prayer. Then those who did not perform the prayer come forward and pray with their leader, performing one prostration. Then their leader leaves after completing his prayer, and each person from the two groups performs one prostration for their own prayer. If the fear is more intense, then men and riders perform one prostration, meaning one unit of prayer."  

قَالَ رَسُولُ اللهِ ﷺ «صَلَاةُ الْخَوْفِ يَقُومُ الْإِمَامُ وَطَائِفَةٌ مِنَ النَّاسِ مَعَهُ فَيَسْجُدُونَ سَجْدَةً وَاحِدَةً وَتَكُونُ طَائِفَةٌ بَيْنَهُمْ وَبَيْنَ الْعَدُوِّ ثُمَّ يَنْصَرِفُ الَّذِينَ سَجَدُوا مَعَ أَمِيرِهِمْ فَيَكُونُ مَكَانُ الَّذِينَ لَمْ يُصَلُّوا وَيَتَقَدَّمُ الَّذِينَ لَمْ يُصَلُّوا فَيُصَلُّونَ مَعَ أَمِيرِهِمْ سَجْدَةً وَاحِدَةً ثُمَّ يَنْصَرِفُ أَمِيرُهُمْ وَقَدْ قَضَى صَلَاتَهُ وَيُصَلِّي كُلُّ وَاحِدَةٍ مِنَ الطَّائِفَتَيْنِ لِصَلَاتِهِ سَجْدَةً لِنَفْسِهِ فَإِنْ كَانَ خَوْفًا أَشَدَّ مِنْ ذَلِكَ فَرِجَالًا وَرُكْبَانًا يَعْنِي بِالسَّجْدَةِ رَكْعَةً »