It is narrated from Sayyiduna Malik bin Abdullah al-Khuza'i (may Allah be pleased with him); he said: "I went on campaign with the Messenger of Allah (peace and blessings be upon him), and we never prayed an obligatory prayer behind an imam who was lighter in prayer than he (peace and blessings be upon him) was."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 16701