The Basic Cause of Misguidance

Shaykh al-Islam Ibn Taymiyyah (may Allah have mercy on him) stated:

The foundation of misguidance among the inhabitants of the earth stems from two things: either regarding as part of religion that which was not legislated by Allah, or regarding as Haraam that which was not forbidden by Allah. Consequently, the basic principle on which Imam Ahmad and other Imams based their schools of fiqh is that:  people’s deeds are divided into two categories: acts of worship that they take as part of their religion, by means of which they may benefit in the Hereafter, or in this world; and customs which are of benefit to them in their daily lives. So, the basic principle concerning acts of worship is that nothing is to be prescribed except that which was legislated by Allah while the basic principle concerning customs is that nothing is to be forbidden except that which was forbidden by Allah.

Source: Sharh Iqtidaa as-Siraat al-Mustaqeem, p. 322 of Shaykh al-‘Uthaymeen