Musannaf Ibn Abi Shayba 3:4779

Ali ibn Mushir narrated to us, from al-Mukhtar ibn Fulful, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) led us in prayer one day. When he finished the prayer, he turned his face toward us and said: "O people, I am your Imam, so do not precede me in bowing, nor in prostration, nor in standing, nor in departing; for indeed, I see you in front of me and behind me."