Musnad Abu Ya'la al-Mawsili 7:20

Al-Hasan ibn Isma'il Abu Sa'id al-Basri narrated to us, Ibrahim ibn Sa'd narrated to us, from his father, from his grandfather, from 'Abd al-Rahman ibn 'Awf: That when the Messenger of Allah (peace and blessings of Allah be upon him) reached 'Abd al-Rahman ibn 'Awf while he was leading the people in prayer, 'Abd al-Rahman intended to step back. The Prophet (peace and blessings of Allah be upon him) gestured to him: "Stay in your place." So he prayed, and the Messenger of Allah (peace and blessings of Allah be upon him) prayed following the prayer of 'Abd al-Rahman.