Sahih Bukhari Volume 002, Book 014, Hadith Number 066.

Narated 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.