Musnad Ahmad 152:1

Muhammad ibn Ja‘far narrated to us, he said: Shu‘bah narrated to us, from Yahya ibn Sa‘id and ‘Abd al-Rahman ibn al-Qasim ibn Muhammad ibn Abi Bakr al-Siddiq, from al-Qasim, from Salih ibn Khawwat, from Sahl ibn Abi Hathmah. As for ‘Abd al-Rahman, he attributed it to the Prophet, may Allah bless him and grant him peace. As for Yahya, he mentioned it from Sahl, who said: "The Imam stands, and a row is behind him, and a row is in front of him. He prays with those who are behind him one rak‘ah and two prostrations. Then he remains standing until they have performed another rak‘ah. Then they move forward to the place of their companions. Then those [others] come and stand in the place of these [first ones]. He prays with them one rak‘ah and two prostrations. Then he sits until they have completed another rak‘ah. Then he performs the taslim upon them."