Musnad Ahmad 27:1891

'Abd al-Razzaq narrated to us, Ma'mar informed us, from al-Zuhri, from Salim, from Ibn 'Umar, who said: "The Messenger of Allah (ﷺ) performed the prayer of fear with one of the two groups for one rak'ah, while the other group was facing the enemy. Then they departed and stood in the place of their companions, facing the enemy. Then those [others] came, and the Prophet (ﷺ) prayed one rak'ah with them, then he performed the taslim. Then these [first ones] completed one rak'ah, and those [second ones] completed one rak'ah."