Musnad Ahmad 25:225

Waki’ narrated to us, Sufyan narrated to us, from Abu Bakr ibn Abi al-Jahm ibn Sukhayr, from Ubayd Allah ibn Abd Allah ibn Utbah, from Ibn Abbas, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) performed the prayer of fear at Dhu Qard—a land from the lands of Banu Sulaym. He arranged the people behind him in two rows: a row parallel to the enemy, and a row behind him. He prayed one rak'ah with the row that was next to him. Then, those [in the first row] retreated to the positions of those [in the second row], and those [in the second row] retreated to the positions of those [in the first row], and he prayed another rak'ah with them."