I came to Zayd bin Thabit and asked him about the prayer of fear. He said, "The Messenger of Allah ﷺ led the prayer with one row standing behind him and another row facing the enemy. He prayed one rakah with them, then they went to take the position of their companions, and the other group came forward. He prayed one rakah with them, then another rakah, and then concluded with the salam. So the Prophet ﷺ prayed two rakahs, while each group prayed one rakah."
أتيت زيد بن ثابت فسألته صلاة الخوف فقال: صلى رسول الله ﷺ وصف خلفه، وصف بإزاء العدو، فصلى بهم ركعة، ثم ذهبوا إلى مصاف إخوانهم، وجاء الآخرون، فصلى بهم ركعة، ثم ركعة، ثم سلم، فكان للنبي ﷺ ركعتان ولكل طائفة ركعة.
Narrated by Nasai (1531), Imam Ahmad (21593), Ibn Khuzayma (1345), Ibn Hibban (2870), and Bayhaqi (3/262), all of them through the route of Sufyan, from Rukayn Fazari, from Qasim bin Hassan, who related it. The wording given is that of Ibn Hibban. As for Nasai, he referred back to the hadith of Huzaifa, Imam Ahmad referred back to the hadith of Ibn Abbas, and Ibn Khuzayma referred back to the hadith of Huzaifa.Its chain of transmission is hasan, for Qasim bin Hassan, who is Amiri Kufi, had two narrators report from him, and Ahmad bin Salih deemed him trustworthy, as Ibn Shahin mentioned in his Thiqat (1094), and Ibn Hibban mentioned him among the trustworthy narrators (5/305). Ibn Khuzayma and Ibn Hibban relied on him and included the hadith in their two Sahihs.As for what Ibn Qattan said, that his status is unknown, in fact his status has become known to those who studied it, and he was deemed trustworthy. Dhahabi said in al-Kashif, "Deemed trustworthy," and he related from Bukhari in al-Mizan that Bukhari said, "His hadith is munkar, and he is unknown," then he mentioned something about him. He cited from his hadith on the authority of Ibn Masud, so perhaps he meant this very hadith. As for Tarikh al-Kabir, Bukhari did not mention anything about him in it, and Ibn Abu Hatim also mentioned him without saying anything about him. Allah knows best.The rest of the narrators in the chain are among the narrators of the Sahih.