Sunan Ibn Majah Book 5, Hadith 1258

حَدَّثَنَا مُحَمَّدُ بْنُ الصَّبَّاحِ، أَنْبَأَنَا جَرِيرٌ، عَنْ عُبَيْدِ اللَّهِ بْنِ عُمَرَ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، قَالَ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ 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 ‏.‏

It was narrated that Ibn ‘Umar said:“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