Ubaidullah bin Abi Ja'far عبيد الله بن أبي جعفر المصري · The Book of al-Bay'ah

"The Messenger of Allah said: 'Whoever obeys me, obeys Allah, and whoever disobeys me, disobeys Allah. Whoever obeys my governor (Amir), he has obeyed me, and whoever disobeys my governor, he has disobeyed me."'

"The Imam is like a shield whose orders should be obeyed when they (the Muslims) fight, and where they should seek protection. If he enjoins fear of Allah and behaves justly, then he will be rewarded, but if he enjoins otherwise, then it will be a burden (of sin) on him."