Musnad Ahmad 830:124

Rawh narrated to us, Ash'ath narrated to us, from al-Hasan, from Abu Bakrah, that he said: "The Prophet, peace and blessings of Allah be upon him, led us in the prayer of fear. He prayed two rak'ahs with some of his companions, then he performed the taslim. They moved back, and others came and stood in their place. He prayed two rak'ahs with them, then he performed the taslim. Thus, there were four rak'ahs for the Prophet, peace and blessings of Allah be upon him, and two rak'ahs for each group."