Shaikh Ibn Uthaymeen said:
Sickness of the heart; (and protection is sought from Allah) it brings about corruption in one’s worldly life and after life. If the heart becomes sick with doubt, polytheism, hypocrisy or a dislike for what Allah has revealed, or hatred towards Allah’s allies, or with anything that is similar to that, then indeed (as a result of that) his worldly life and after life will end up in ruin. Therefore, it is binding upon you whenever you ask for well-being to bring to mind that you’re asking (for both) well-being against illness of the heart as well as illness of the body.
Sharh Ar-Riyaadhus Saaliheen Volume 6 Page 21
Translated by Abu Arwa Ali bin Mushtaq Mir