Shaykh al-Islām—may Allah Most High have mercy on him—said:
Section: “Fanā” (Passing Away)
The term fanā, as it occurs in Sufi writings, is explained in three ways.
(The first): The heart’s ceasing to direct its desire, reliance, worship, and related acts toward anything besides the Lord. This is true and sound; it is pure monotheism and sincerity. In “reality,” it is the heart’s worship, reliance, seeking help, adoration, repentant return, and turning toward Allah alone, without partner, together with the knowledge and spiritual states that follow from this. No one is exempt from this.
This is the “sound heart” of which Allah says:
“Except one who comes to Allah with a sound heart.”
It means the heart’s freedom from corrupt beliefs, corrupt desires, and all that follows from them.