Hadith - Sahih Al-Bukhari 9:265
Allah's Messenger SAW said, "If any ruler having the authority to rule Muslim subjects dies while he is deceiving them, Allah will forbid Paradise for him."
Sahih Muslim - Volume 9, Book 89, Number 264, Narrated Ma'qil
I heard the Prophet (saaws) saying, "Any man whom Allah has given the authority of ruling some people and he does not look after them in an honest manner, will never feel even the smell of Paradise."
If an Imam (leader) or Amir (ruler) becomes corrupt he should first be given a call to Islam (submission) in private, or possibly in public if his evil deeds were done in public. If he does not turn away from his evil deeds, he should be overthrown or removed from position. However, in the process of removing him from position, he should not be physically fought, such as waging war with weapons. This is because the ruler is still muslim, and the muslims are not to attack or kill another muslim. If, however, the Muslim leader or ruler completely abandons his salat, he nullifies his Islam and can be fought if necessary. (However, to prove this, it would take an Islamic court or similar situation in which the person could defend themselves against all accusations).