Mishkat al-Masabih 1420
Salim b. ‘Abdallah b. ‘Umar quoted his father as saying:
I went on an expedition with God’s Messenger to Najd, and when we came in front of the enemy we drew up in line facing them. God’s Messenger then stood up and led us in prayer, and one section stood up along with him while another faced the enemy. He prayed a rak'a with those who were with him and made two prostrations, and then they changed places with those who had not prayed. When they came God’s Messenger prayed a rak'a with them and made two prostrations, then he uttered the salutation and each of them got up and left, after which he prayed a rak'a alone and made two prostrations. Nafi‘ transmitted something similar, adding that when there was greater cause for fear than on that occasion they prayed standing on their feet or mounted, without considering whether or not they were facing the qibla . Nafi‘ said he thought God’s Messenger was the one on whose authority Ibn ‘Umar mentioned that. 

Bukhari transmitted it.
عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ عَنْ أَبِيهِ قَالَ: غَزَوْتُ مَعَ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قِبَلَ نَجْدٍ فَوَازَيْنَا الْعَدُوَّ فَصَافَفْنَا لَهُمْ فَقَامَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُصَلِّي لَنَا فَقَامَتْ طَائِفَةٌ مَعَهُ وَأَقْبَلَتْ طَائِفَةٌ عَلَى الْعَدُوِّ وَرَكَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِمَنْ مَعَهُ وَسَجَدَ سَجْدَتَيْنِ ثُمَّ انْصَرَفُوا مَكَانَ الطَّائِفَةِ الَّتِي لم تصل فجاؤوا فَرَكَعَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بهم رَكْعَةً وَسَجَدَ سَجْدَتَيْنِ وَرَوَى نَافِعٌ نَحْوَهُ وَزَادَ: فَإِن كَانَ خوف هُوَ أَشَدُّ مِنْ ذَلِكَ صَلَّوْا رِجَالًا قِيَامًا عَلَى أَقْدَامِهِمْ أَوْ رُكْبَانًا مُسْتَقْبِلِي الْقِبْلَةِ أَوْ غَيْرَ مُسْتَقْبِلِيهَا قَالَ نَافِعٌ: لَا أُرَى ابْنَ عُمَرَ ذَكَرَ ذَلِكَ إِلَّا عَنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ. رَوَاهُ البُخَارِيّ