Musnad Abu Ya'la al-Mawsili 128:13

Abu Ma'mar Isma'il ibn Ibrahim al-Hudhali narrated to us, Isma'il narrated to us, from Sharik ibn Abi Namir, from Anas, who said: I have never prayed behind an Imam who was lighter in prayer than the Messenger of Allah, may Allah bless him and grant him peace.