Musnad Ahmad 1034:112

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, Abu Rawq 'Atiyyah ibn al-Harith narrated to us, Makhmal ibn Damath narrated to us, he said: I went on a military expedition with Sa'id ibn al-'As. He said: The people asked, "Who among you witnessed the Fear Prayer (Salat al-Khawf) with the Messenger of Allah, may Allah bless him and grant him peace?" Hudhayfah said: "I did. He prayed one rak'ah with a group of the people, while a group faced the enemy. Then those went and stood in the place of their companions facing the enemy, and the other group came, and the Messenger of Allah, may Allah bless him and grant him peace, prayed one rak'ah with them. Then he performed the taslim. Thus, the Messenger of Allah, may Allah bless him and grant him peace, had two rak'ahs, and each group had one rak'ah."