Jami` at-Tirmidhi 565

Sahl bin Abi Hathmah said about Salat Al-Khawf:"The Imam stands facing the Qiblah while a group of them stand with him, and a group is before the enemy, facing the enemy. He leads them in a Rak'ah, and they perform a bowing by themselves, and they perform two prostrations in their places. Then they go to take the position of the others and the others come (for prayers). He (the Imam) bows for one Rak'ah with them and performs two prostrations with them. That is two for him and one for them, then they perform one bowing and two prostrations