The Messenger of Allah ﷺ prayed at Dhi Qarad and the people formed two rows behind him, one row behind him and one row facing the enemy. He led those who were behind him in praying one rak'ah, then they went and took the place of the others, and the others came and he led them in praying one rak'ah, and they did not make it up. (Using translation from Nasāʾī 1533)  

أَنَّ رَسُولَ اللهِ ﷺ «صَلَّى بِذِي قَرَدٍ فَصَفَّ النَّاسَ خَلْفَهُ صَفَّيْنِ صَفٌّ خَلْفَهُ وَصَفٌّ مُصَافُّو الْعَدُوِّ فَصَلَّى بِالَّذِينَ خَلْفَهُ رَكْعَةً ثُمَّ انْصَرَفَ هَؤُلَاءِ إِلَى مَكَانِ هَؤُلَاءِ وَجَاءَ أُولَئِكَ فَصَلَّى بِهِمْ رَكْعَةً وَلَمْ يَقْضُوا»  

The Messenger of Allah ﷺ led the people in offering the fear prayer, two rak'ahs. Then he said the taslim and led others in offering the fear prayer, then he said the taslim. So the Prophet ﷺ had prayed four rak'ahs. (Using translation from Nasāʾī 1551)   

أَنَّ رَسُولَ اللهِ ﷺ «صَلَّى بِالْقَوْمِ فِي الْخَوْفِ رَكْعَتَيْنِ ثُمَّ سَلَّمَ ثُمَّ صَلَّى بِالْقَوْمِ الْآخَرِينَ رَكْعَتَيْنِ ثُمَّ سَلَّمَ فَصَلَّى النَّبِيُّ ﷺ أَرْبَعًا» خَالَفَهُ يُونُسُ بْنُ عُبَيْدٍ  

[Machine] The Messenger of Allah ﷺ performed the prayer of fear with his companions. He prayed two units of prayer and the others faced their enemies. Then he saluted and the others came and he prayed two units with them and then saluted. So the Messenger of Allah ﷺ prayed four units and the people prayed two units each.  

أَنَّ رَسُولَ اللهِ ﷺ «صَلَّى بِأَصْحَابِهِ صَلَاةَ الْخَوْفِ فَصَلَّى بِطَائِفَةٍ رَكْعَتَيْنِ وَالْآخَرُونَ يُقْبِلُونَ عَلَى عَدُوِّهِمْ ثُمَّ سَلَّمَ ثُمَّ جَاءَ الْآخَرُونَ فَصَلَّى بِهِمْ رَكْعَتَيْنِ ثُمَّ سَلَّمَ فَكَانَتْ لِرَسُولِ اللهِ ﷺ أَرْبَعَ رَكَعَاتٍ وَلِلنَّاسِ رَكْعَتَيْنِ رَكْعَتَيْنِ»  

"The prayer of the resident was enjoined on the tongue of your Prophet ﷺ, four (rak'ahs), and the prayer of the traveler is two rak'ahs, and the prayer of fear is one rak'ah." (Using translation from Nasāʾī 1441)  

«فُرِضَتْ صَلَاةُ الْحَضَرِ عَلَى لِسَانِ نَبِيِّكُمْ أَرْبَعًا وَصَلَاةُ السَّفَرِ رَكْعَتَيْنِ وَصَلَاةُ الْخَوْفِ رَكْعَةً»