Musnad Ahmad 1034:213

Yahya ibn Adam narrated to us, Isra’il narrated to us, from Abu Ishaq, from Sulaym ibn Abd al-Saluli, who said: We were with Sa’id ibn al-‘As in Tabaristan, and with him were a group of the companions of the Messenger of Allah, may Allah bless him and grant him peace. He said: "Which of you prayed the Fear Prayer with the Messenger of Allah, may Allah bless him and grant him peace?" Hudhayfah said: "I did. Command your companions to stand in two groups: one group behind you, and one group facing the enemy. You say the Takbir, and they all say the Takbir together. Then you bow, and they all bow together. Then you rise, and they all rise together. Then you prostrate, and the group that is next to you prostrates with you, while the group that is facing the enemy remains standing facing the enemy. When you raise your head from the prostration, they prostrate. Then these move back, and the others move forward, and they stand in their ranks. Then you bow, and they all bow together. Then you rise, and they all rise together. Then you prostrate, and the group that is next to you prostrates, while the other group remains standing facing the enemy. When you raise your head from the prostration, they prostrate. Then you say the Taslim, and they say the Taslim to one another. And command your companions that if they are stirred by a stirring from the enemy, then fighting and speaking have become permissible for them."