Muwatta Malik, Book 11, Hadith 3 (USC-MSA) ⓘ

Yahya related to me from Malik from Nafi that Abdullah ibn Umar, when asked about the fear prayer said, "The imam and a group of people go forward and the imam prays a raka with them, while another group, who have not yet prayed, position themselves between him and the enemy. When those who are with him have prayed a raka they draw back to where those who have not prayed are, and do not say the taslim. Then those who have not prayed come forward and pray a raka with him. Then the imam leaves, as he has now prayed two rakas. Everyone else in the two groups stands and prays a raka by himself after the imam has left. In this way each of the two groups will have prayed two rakas. If the fear is greater than that, then the men pray standing on their feet or mounted, either facing the qibla or otherwise."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ إِذَا سُئِلَ عَنْ صَلاَةِ الْخَوْفِ، قَالَ يَتَقَدَّمُ الإِمَامُ وَطَائِفَةٌ مِنَ النَّاسِ فَيُصَلِّي بِهِمُ الإِمَامُ رَكْعَةً وَتَكُونُ طَائِفَةٌ مِنْهُمْ بَيْنَهُ وَبَيْنَ الْعَدُوِّ لَمْ يُصَلُّوا فَإِذَا صَلَّى الَّذِينَ مَعَهُ رَكْعَةً اسْتَأْخَرُوا مَكَانَ الَّذِينَ لَمْ يُصَلُّوا وَلاَ يُسَلِّمُونَ وَيَتَقَدَّمُ الَّذِينَ لَمْ يُصَلُّوا فَيُصَلُّونَ مَعَهُ رَكْعَةً ثُمَّ يَنْصَرِفُ الإِمَامُ وَقَدْ صَلَّى رَكْعَتَيْنِ فَتَقُومُ كُلُّ وَاحِدَةٍ مِنَ الطَّائِفَتَيْنِ فَيُصَلُّونَ لأَنْفُسِهِمْ رَكْعَةً رَكْعَةً بَعْدَ أَنْ يَنْصَرِفَ الإِمَامُ فَيَكُونُ كُلُّ وَاحِدَةٍ مِنَ الطَّائِفَتَيْنِ قَدْ صَلَّوْا رَكْعَتَيْنِ فَإِنْ كَانَ خَوْفًا هُوَ أَشَدَّ مِنْ ذَلِكَ صَلَّوْا رِجَالاً قِيَامًا عَلَى أَقْدَامِهِمْ أَوْ رُكْبَانًا مُسْتَقْبِلِي الْقِبْلَةِ أَوْ غَيْرَ مُسْتَقْبِلِيهَا ‏.‏

Malik said that Nafi said, "I do not believe that Abdullah ibn Umar related it from anyone other than the Messenger of Allah, may Allah bless him and grant him peace."