Musnad Ahmad 27:1971

Yahya ibn Adam narrated to us, Sufyan narrated to us, from Musa ibn Uqbah, from Nafi’, from Ibn Umar, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed the prayer of fear on one of his days. A group stood with him, and a group was facing the enemy. He prayed one rak'ah with those who were with him, then they went and the others came, and he prayed one rak'ah with them. Then both groups completed one rak'ah each."