Musnad Ahmad 33:68

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Hakam, from Yazid al-Faqir, from Jabir ibn Abdullah: "The Messenger of Allah (ﷺ) led them in the prayer of fear. A row stood in front of him, and a row stood behind him. He prayed one rak'ah and two prostrations with those who were behind him. Then, those [in the front] moved forward until they stood in the place of their companions, and those [who were behind] came until they stood in the place of those [who had moved]. The Messenger of Allah (ﷺ) then prayed one rak'ah and two prostrations with them, then he performed the taslim. Thus, there were two rak'ahs for the Prophet (ﷺ), and one rak'ah for each of them."