Sulaym bin Abd al-Saluli (may Allah have mercy on him) said: I was with Sa'id bin al-As (may Allah be pleased with him) in Tabaristan, and Sa'id (may Allah be pleased with him) said: "Who among you was present with the Prophet (peace and blessings be upon him) at the fear prayer?" Sayyiduna Hudhayfah (may Allah be pleased with him) said: "I was present." He described the prayer of the Prophet (peace and blessings be upon him) that was prayed at Usfan. In the narration of Tha'labah, the narrator's words 'a row facing the enemy' refer to the state of prostration, and his words 'they went to their place' mean that, after finishing the first rak'ah, the front row came back and the rear row went forward. In this way both rak'ahs were prayed with the imam, and there is no need to complete the second rak'ah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6047