Sahih Bukhari Volume 5, Book 64, Hadith 4135

حَدَّثَنَا مُسَدَّدٌ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ سَالِمِ بْنِ عَبْدِ اللَّهِ بْنِ عُمَرَ، عَنْ أَبِيهِ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم 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

Narrated By 'Abdullah bin 'Umar : Allah's Apostle 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 Rak'a 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 Rak'a.