Musnad Ahmad 26:332

'Abd al-Razzaq narrated to us, Sufyan informed us, from Khasif, from Abu 'Ubaydah, from 'Abdullah, who said: We were with the Messenger of Allah (peace and blessings of Allah be upon him), and he arranged a row behind him, and a row parallel to the enemy. He said: They were all in prayer. He said: He said the takbir, and they all said the takbir. He prayed one rak'ah with the row that was next to him, while the row parallel to the enemy remained. He said: Then these went, and those came, and he prayed one rak'ah with them. Then those who had prayed the second rak'ah with him stood up and completed it where they were. Then these went to the position of those, and those came and completed one rak'ah.