Sahih Muslim Book - 6, Hadith - 1944

وَحَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِى شَيْبَةَ حَدَّثَنَا يَحْيَى بْنُ آدَمَ عَنْ سُفْيَانَ عَنْ مُوسَى بْنِ عُقْبَةَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ قَالَ صَلَّى رَسُولُ اللَّهِ -صلى الله عليه وسلم- صَلاَةَ الْخَوْفِ فِى بَعْضِ أَيَّامِهِ فَقَامَتْ طَائِفَةٌ مَعَهُ وَطَائِفَةٌ بِإِزَاءِ الْعَدُوِّ فَصَلَّى بِالَّذِينَ مَعَهُ رَكْعَةً ثُمَّ ذَهَبُوا وَجَاءَ الآخَرُونَ فَصَلَّى بِهِمْ رَكْعَةً ثُمَّ قَضَتِ الطَّائِفَتَانِ رَكْعَةً رَكْعَةً - قَالَ - وَقَالَ ابْنُ عُمَرَ فَإِذَا كَانَ خَوْفٌ أَكْثَرَ مِنْ ذَلِكَ فَصَلِّ رَاكِبًا أَوْ قَائِمًا تُومِئُ إِيمَاءً.

It was narrated that Ibn 'Umar said: "The Messenger of Allah (s.a.w) prayed the fear prayer at some times. One group stood with him while the other faced the enemy. He led those who were with him in praying one Rak'ah, then they left and the other came, and he led them in praying one Rak'ah. Then both groups made up one Rak'ah each." Ibn 'Umar said: "And if the fear is worse than that, then pray riding or standing, by means of gestures."