Every religion has their on Holy book and they follow them. Like in Hindus there are Gita, Ramayana, Mahabharata and Vedam are the epic, Christians has Holy Bible so that Islam has Koran. They follow it strictly. Have you ever think Why do Islamic believers follow the Koran ?
The Koran is the holy book of the Islamic religion. It contains the words of Allah as revealed to Mohammed by the archangel Gabriel in a series of verses describing the ways in which Muslims should conduct their lives. It specifies daily prayers. And emphasizes the need for brotherly love and charity between Muslims. Although Muslims do not worship Mohammed, they show him the greatest respect. They believe that the Koran is the word of Allah and was not composed by Mohammed.
By following Koran they show respect to their Allah or God and by doing this they feel peace.