Abu Dawud Book 2, Hadith Number 1239.
Chapter : A sixth view that the imam should lead each section in one rakah and then utter the salutation. thereafter those behind him should complete their second rakah by themselves. the other section then should take their place and offer one rakah.
Narated By Abdullah ibn Mas’ud : The Apostle of Allah (PBUH) led us in prayer in the time of danger. They (the people) stood in two rows. One row was behind the Apostle of Allah (PBUH) and the other faced the enemy. The Apostle of Allah (PBUH) led them in one rak’ah, and then the other section came and took their place; they went and faced the enemy. The Prophet (PBUH) led them in one rak’ah and uttered the salutation. They stood up and prayed the second rak’ah by themselves and uttered the salutation and went away; they took the place of the other section facing the enemy. They came back and took their place. They prayed one rak’ah by themselves and then uttered the salutation.