Musannaf Ibn Abi Shayba 3:5907

Yahya ibn Adam narrated to us, he said: Sufyan narrated to us, from Musa ibn Uqbah, from Nafi’, from Ibn Umar, who said: The Messenger of Allah (may Allah bless him and grant him peace) performed the prayer of fear on one of his days. A group stood with him, and a group stood facing the enemy. He prayed one rak'ah with those who were with him, then they left, and the others came, and he prayed one rak'ah with them. Then both groups completed one rak'ah each. He said: And Ibn Umar said: If the fear is greater than that, then pray while riding or standing, gesturing with a nod.