Abu Dawud Book 2, Hadith Number 1233.

Chapter : A second view goes that the people should form a row along with the imam and form the other row facing the enemy. the imam should lead the people who are with him in one rakah and then should remain standing so that those who are with him might complete the other rakah. these people should then turn away and form a row facing the enemy. the other section should then come and the imam should lead them in one rakah and he would remain seated in his place until they complete the other rakah by themselves. the imam should then utter the salutation along with all of them.

Narated By Sahl b. Abi Hathmah : The Prophet (PBUH) prayed in the times of danger and divided them (the people) behind him in two rows. He then led those who were near him in one rak’ah. Then he stood and remained standing till those who were in the second row offered one rak’ah. Thereafter they came forward and those who were in front of them (in the first row) stepped backward. The Prophet (PBUH) led them in one rak’ah of prayer. He sat down till those who were in the second row completed one rak’ah. He then uttered the salutation.