Musnad Ahmad 1034:192

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Abu Ishaq, who said: Someone who was with Sa'id ibn al-'As in a military expedition called the Expedition of al-Khashab, and Hudhayfah ibn al-Yaman was with him, narrated to me. Sa'id said: "Which of you witnessed the Fear Prayer (Salat al-Khawf) with the Messenger of Allah, peace and blessings of Allah be upon him?" Hudhayfah said: "I did." He said: So Hudhayfah commanded them, and they put on their weapons. Then he said: "If an agitation stirs you, then fighting has become permissible for you." He said: "He prayed one rak'ah with one of the two groups, while the other group was facing the enemy. Then those departed and stood in the place of the others, and those came, and he prayed another rak'ah with them, then he performed the taslim upon them."