Bulugh al-Maram 477
The Book of Prayer
Narrated Jabir (RA): "I witnessed Salat-al-Khauf with Allah's Messenger (ﷺ) and we lined up in two rows behind him with the enemy between us and the Qiblah. The Prophet (ﷺ) uttered the Takbir and we all uttered it; then he bowed and we all bowed; then he raised his head after bowing and we all raised ours; then he and the row immediately behind him went down in prostration while the rear row stood facing the enemy. Then, when Allah's Messenger (ﷺ) finished the prostration and the row immediately behind him stood up..." then he mentioned the Hadith. And in a narration "Then he prostrated and the first row prostrated with him. So when they stood, the second row prostrated then the first row went back and the second row came forward..." then he mentioned a similar to it. And at the end of it "Then the Prophet (ﷺ) uttered the Taslim (salutation) and we all did so." [Reported by Muslim].