Messenger of Allah ﷺ led the Fear-prayer with one of the two batches of the army while the other (batch) faced the enemy. Then the first batch went away and took places of their companions (i.e. second batch) and the second batch came and he led his second rakʿa with them. Then he (i.e. the Prophet: finished his prayer with Taslim and then each of the two batches got up and completed their remaining one rakʿa.  

أَنَّ رَسُولَ اللَّهِ ﷺ صَلَّى بِإِحْدَى الطَّائِفَتَيْنِ وَالطَّائِفَةُ الأُخْرَى مُوَاجِهَةُ الْعَدُوِّ ثُمَّ انْصَرَفُوا فَقَامُوا فِي مَقَامِ أَصْحَابِهِمْ فَجَاءَ أُولَئِكَ فَصَلَّى بِهِمْ رَكْعَةً ثُمَّ سَلَّمَ عَلَيْهِمْ ثُمَّ قَامَ هَؤُلاَءِ فَقَضَوْا رَكْعَتَهُمْ وَقَامَ هَؤُلاَءِ فَقَضَوْا رَكْعَتَهُمْ  

(describing the Fear prayer): The Imam stands up facing the Qibla and one batch of them (i.e. the army) (out of the two) prays along with him and the other batch faces the enemy. The Imam offers one rakʿa with the first batch they themselves stand up alone and offer one bowing and two prostrations while they are still in their place, and then go away to relieve the second batch, and the second batch comes (and takes the place of the first batch in the prayer behind the Imam) and he offers the second rakʿa with them. So he completes his two-rakʿat and then the second batch bows and prostrates two prostrations (i.e. complete their second rakʿa and thus all complete their prayer). (This hadith has also been narrated through two other chain by Sahl b. Abi Hathma)  

يَقُومُ الإِمَامُ مُسْتَقْبِلَ الْقِبْلَةِ وَطَائِفَةٌ مِنْهُمْ مَعَهُ وَطَائِفَةٌ مِنْ قِبَلِ الْعَدُوِّ وُجُوهُهُمْ إِلَى الْعَدُوِّ فَيُصَلِّي بِالَّذِينَ مَعَهُ رَكْعَةً ثُمَّ يَقُومُونَ فَيَرْكَعُونَ لأَنْفُسِهِمْ رَكْعَةً وَيَسْجُدُونَ سَجْدَتَيْنِ فِي مَكَانِهِمْ ثُمَّ يَذْهَبُ هَؤُلاَءِ إِلَى مَقَامِ أُولَئِكَ فَيَرْكَعُ بِهِمْ رَكْعَةً فَلَهُ ثِنْتَانِ ثُمَّ يَرْكَعُونَ وَيَسْجُدُونَ سَجْدَتَيْنِ  

Concerning those who witnessed the Fear Prayer that was performed in the battle of Dhat-ur-Riqa' in the company of Messenger of Allah ﷺ; One batch lined up behind him while another batch (lined up) facing the enemy. The Prophet ﷺ led the batch that was with him in one rakʿa, and he stayed in the standing posture while that batch completed their (two rakʿat) prayer by themselves and went away, lining in the face of the enemy, while the other batch came and he (i.e. the Prophet) offered his remaining rakʿa with them, and then, kept on sitting till they completed their prayer by themselves, and he then finished his prayer with Taslim along with them.  

عَنْ صَالِحِ بْنِ خَوَّاتٍ عَمَّنْ شَهِدَ رَسُولَ اللَّهِ ﷺ يَوْمَ ذَاتِ الرِّقَاعِ صَلَّى صَلاَةَ الْخَوْفِ أَنَّ طَائِفَةً صَفَّتْ مَعَهُ وَطَائِفَةٌ وُجَاهَ الْعَدُوِّ فَصَلَّى بِالَّتِي مَعَهُ رَكْعَةً ثُمَّ ثَبَتَ قَائِمًا وَأَتَمُّوا لأَنْفُسِهِمْ ثُمَّ انْصَرَفُوا فَصَفُّوا وُجَاهَ الْعَدُوِّ وَجَاءَتِ الطَّائِفَةُ الأُخْرَى فَصَلَّى بِهِمِ الرَّكْعَةَ الَّتِي بَقِيَتْ مِنْ صَلاَتِهِ ثُمَّ ثَبَتَ جَالِسًا وَأَتَمُّوا لأَنْفُسِهِمْ ثُمَّ سَلَّمَ بِهِمْ  

"We were in the company of the Prophet (during the battle of) Dhat-ur-Riqa', and we came across a shady tree and we left it for the Prophet (to take rest under its shade). A man from the pagans came while the Prophet's sword was hanging on the tree. He took it out of its sheath secretly and said (to the Prophet ﷺ ), 'Are you afraid of me?' The Prophet ﷺ said, 'No.' He said, 'Who can save you from me?' The Prophet ﷺ said, Allah.' The companions of the Prophet ﷺ threatened him, then the Iqama for the prayer was announced and the Prophet ﷺ offered a two rakʿat Fear prayer with one of the two batches, and that batch went aside and he offered two rakʿa-t with the other batch. So the Prophet ﷺ offered four rakʿat but the people offered two rakʿat only." (The subnarrator) Abu Bishr added, "The man was Ghaurath bin Al-Harith and the battle was waged against Muharib Khasafa."  

كُنَّا مَعَ النَّبِيِّ ﷺ بِذَاتِ الرِّقَاعِ فَإِذَا أَتَيْنَا عَلَى شَجَرَةٍ ظَلِيلَةٍ تَرَكْنَاهَا لِلنَّبِيِّ ﷺ فَجَاءَ رَجُلٌ مِنَ الْمُشْرِكِينَ وَسَيْفُ النَّبِيِّ ﷺ مُعَلَّقٌ بِالشَّجَرَةِ فَاخْتَرَطَهُ فَقَالَ تَخَافُنِي قَالَ لاَ قَالَ فَمَنْ يَمْنَعُكَ مِنِّي قَالَ اللَّهُ فَتَهَدَّدَهُ أَصْحَابُ النَّبِيِّ ﷺ وَأُقِيمَتِ الصَّلاَةُ فَصَلَّى بِطَائِفَةٍ رَكْعَتَيْنِ ثُمَّ تَأَخَّرُوا وَصَلَّى بِالطَّائِفَةِ الأُخْرَى رَكْعَتَيْنِ وَكَانَ لِلنَّبِيِّ ﷺ أَرْبَعٌ وَلِلْقَوْمِ رَكْعَتَيْنِ وَقَالَ مُسَدَّدٌ عَنْ أَبِي عَوَانَةَ عَنْ أَبِي بِشْرٍ اسْمُ الرَّجُلِ غَوْرَثُ بْنُ الْحَارِثِ وَقَاتَلَ فِيهَا مُحَارِبَ خَصَفَةَ