Musnad Ahmad 25:524

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Dawud ibn al-Husayn, the freed slave of 'Amr ibn 'Uthman, narrated to me, from 'Ikrimah, the freed slave of Ibn 'Abbas, from Ibn 'Abbas, who said: "The prayer of fear was nothing but like the prayer of these guards of yours today behind your Imams, except that it was in turns. A group stood while they were all with the Messenger of Allah, may Allah bless him and grant him peace, and a group prostrated with him. Then the Messenger of Allah, may Allah bless him and grant him peace, stood, and those who were standing prostrated for themselves. Then the Messenger of Allah, may Allah bless him and grant him peace, stood, and they all stood with him. Then he bowed, and they all bowed with him. Then he prostrated, so those who were standing with him the first time prostrated, and the others who had prostrated with him the first time stood. When the Messenger of Allah, may Allah bless him and grant him peace, sat, along with those who had prostrated with him at the end of their prayer, those who were standing prostrated for themselves, then they sat. Then the Messenger of Allah, may Allah bless him and grant him peace, gathered them with the Taslim."