Musnad Ahmad 1018:75

Ishaq ibn Isa narrated to us, Malik informed me, from Yazid ibn Ruman, from Salih ibn Khawwat ibn Jubayr, from one who prayed the Fear Prayer with the Messenger of Allah (peace and blessings of Allah be upon him) on the day of Dhat al-Riqa: That a group lined up with him, and a group stood facing the enemy. He prayed one rak'ah with the group that was with him, then he remained standing while they completed [the prayer] for themselves. Then they departed and lined up facing the enemy. The other group came, and he prayed the remaining rak'ah of his prayer with them. Then he remained sitting while they completed [the prayer] for themselves, then he performed the taslim. Malik said: This is the most beloved of what I have heard regarding the Fear Prayer.