Musnad Abu Ya'la al-Mawsili 159:93

And with his chain of narration from the Prophet, may Allah bless him and grant him peace, he said: "The Imam is only a shield, behind whom one fights and by whom one is protected. If he fears Allah and acts justly, he will have a reward for that. And if he commands otherwise, then upon him is the burden of that. And the Imam is only to be followed."