Musnad Ahmad 25:1505

'Abd al-Rahman narrated to us, Sufyan narrated to us, from Abu Bakr—meaning Ibn Abi al-Jahm—from 'Ubayd Allah ibn 'Abd Allah, from Ibn 'Abbas, who said: "The Messenger of Allah (ﷺ) performed the Fear Prayer at Dhu Qard. One row stood behind him, and one row stood parallel to the enemy. He prayed one rak'ah with them. Then those went to the position of those, and those came, and he prayed one rak'ah with them. Then he performed the taslim. Thus, there were two rak'ahs for the Prophet (ﷺ), and one rak'ah for each group."