Sahih Bukhari Volume 2, Book 12, Hadith 945

حَدَّثَنَا حَيْوَةُ بْنُ شُرَيْحٍ، قَالَ حَدَّثَنَا مُحَمَّدُ بْنُ حَرْبٍ، عَنِ الزُّبَيْدِيِّ، عَنِ الزُّهْرِيِّ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ قَامَ النَّبِيُّ صلى الله عليه وسلم وَقَامَ النَّاسُ مَعَهُ، فَكَبَّرَ وَكَبَّرُوا مَعَهُ، وَرَكَعَ وَرَكَعَ نَاسٌ مِنْهُمْ، ثُمَّ سَجَدَ وَسَجَدُوا مَعَهُ، ثُمَّ قَامَ لِلثَّانِيَةِ فَقَامَ الَّذِينَ سَجَدُوا وَحَرَسُوا إِخْوَانَهُمْ، وَأَتَتِ الطَّائِفَةُ الأُخْرَى فَرَكَعُوا وَسَجَدُوا مَعَهُ، وَالنَّاسُ كُلُّهُمْ فِي صَلاَةٍ، وَلَكِنْ يَحْرُسُ بَعْضُهُمْ بَعْضًا‏

Narrated By Ibn Abbas : Once the Prophet (p.b.u.h) led the fear prayer and the people stood behind him. He said Takbir (Allahu-Akbar) and the people said the same. He bowed and some of them bowed. Then he prostrated and they also prostrated. Then he stood for the second Raka and those who had prayed the first Raka left and guarded their brothers. The second party joined him and performed bowing and prostration with him. All the people were in prayer but they were guarding one another during the prayer.