Musnad Ahmad 1034:28

Waki’ narrated to us, Sufyan narrated to us, from Abu Bakr ibn Abi al-Jahm, 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 belonging to the Banu Sulaym. He arranged the people behind him in two rows: one row parallel to the enemy, and one 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 these [in the second row], and those [in the second row] retreated to the positions of these [in the first row], and he prayed another rak'ah with them."