Musnad Abu Ya'la al-Mawsili 159:77

And with his chain of narration, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "The Imam is only a shield; one fights from behind him and is protected by him. So if he commands the fear of Allah and acts justly, then he has a reward for that. And if he commands other than that, then upon him is the consequence of it."