Musnad Ahmad 27:1699

Abu al-Mughirah narrated to us, al-Awza'i narrated to us, from Ayyub ibn Musa, from Nafi', from Ibn Umar: "The Prophet, peace and blessings of Allah be upon him, performed the prayer of fear with one of the two groups, [performing] one rak'ah and two prostrations, while the other group was facing the enemy. Then the group that was with the Prophet, peace and blessings of Allah be upon him, departed, and the other group came forward. The Prophet, peace and blessings of Allah be upon him, prayed one rak'ah and two prostrations with them. Then the Prophet, peace and blessings of Allah be upon him, performed the taslim. Then every man from the two groups stood up and performed one rak'ah and two prostrations for himself."