The origin of Muslim empire took place during the 7th century CE, when Arab Muslim armies started to subdue Byzantine, Sasanian, and other empires. In approximately 20 years the Muslim empire expanded to three continents. However, this expansion was motivated more by political reasons than religious. Nevertheless, Muslim invaders shared their religious beliefs with the captivated nations.
The system of laws in Islam is called Sharia and contains directions from Mohammed and are written in Koran. Sharia was compiled during the life of Mohammed.