What are the sins of the heart?

There are two kinds of action from which one should abstain, disbelief and disobedience. The major disobedience of the heart includes lack of sincerity, vanity, pride, boasting, haughtiness and despair of Allah's mercy.

Hafiz Ibn Qayyim rahimahullah writes that examples of disbelief include having doubts, hypocrisy and polytheism. Disobedience can be major or minor.

Major disobedience is lack of sincerity (riya), vanity ('ujub), pride (kibr), boasting (fakhr), haughtiness (khuyala), feeling hopeless of the mercy of Allah (qunut), to not fear the punishment of Allah, feeling pleasure when a Muslim is harmed, expressing joy when a Muslim is afflicted with trouble, spreading indecency among Muslims, and being jealous of Muslims, “and other such sins that are worse than adultery and drinking wine etc.”

He also writes, “If the heart is not cleansed then it will become foul, which will affect the entire body.”