Allaah (The Most High) said:

فِي قُلُوبِهِم مَّرَضٌ فَزَادَهُمُ اللَّهُ مَرَضًا

In their hearts is a disease and Allah has increased their disease. [2:10]

The disease of the heart (appears) when it is isolated from its soundness and uprightness, for indeed its soundness is when it is acquainted with the truth, has love for it and prefers it over other (affairs). Its sickness is either due to (being afflicted) with doubt or preferring it over (the truth).

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[Source: Badaa’i At-Tafseer Al-Jaami Limaa Fassarahu Al-Imaam Ibnu Qayyim’ Vol 1. page 95]