Salim b. Abdullah b. 'Umar reported: The Messenger of Allah (may peace he upon him) led on* of the two groups In one rak'ah of prayer in danger, while the other group faced the -enemy. Then they (the members of the first group) went back and replaced their companions who were facing the enemy. and then they (the members of the second group) came and the Apostle of Allah ﷺ led them in one rak'ah of prayer. Then the Apostle of Allah ﷺ pronounced salutation, and then they (the members of the Ant group) completed the rak'ah and they (the members of the second group) completed the rak'ah. This hadith has been narrated by another chain of tranamitters.

حَدَّثَنَا عَبْدُ بْنُ حُمَيْدٍ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ سَالِمٍ، عَنِ ابْنِ عُمَرَ، قَالَ صَلَّى رَسُولُ اللَّهِ ﷺ صَلاَةَ الْخَوْفِ بِإِحْدَى الطَّائِفَتَيْنِ رَكْعَةً وَالطَّائِفَةُ الأُخْرَى مُوَاجِهَةُ الْعَدُوِّ ثُمَّ انْصَرَفُوا وَقَامُوا فِي مَقَامِ أَصْحَابِهِمْ مُقْبِلِينَ عَلَى الْعَدُوِّ وَجَاءَ أُولَئِكَ ثُمَّ صَلَّى بِهِمُ النَّبِيُّ ﷺ رَكْعَةً ثُمَّ سَلَّمَ النَّبِيُّ ﷺ ثُمَّ قَضَى هَؤُلاَءِ رَكْعَةً وَهَؤُلاَءِ رَكْعَةً .