Malik b. Dinar said: “A slave has never been punished with anything more severe than hardness of the heart.”
The importance of this topic is highlighted by the supplication of the Prophet (sallAllahu alaihi wa sallam) where he sought refuge with Allah from the hardness of the heart:
“O Allah, I seek refuge with You from feebleness, laziness, stinginess, cowardice, senility, hardness of the heart], heedlessness, destitution, humiliation and privation.
And I seek refuge with You from poverty, disbelief, shirk, disobedience, opposition to the truth, hypocrisy, seeking fame and showing off. And I seek refuge with You from deafness, dumbness, insanity, leukoderma, leprosy, and all chronic diseases.”
Al-Allamah al-Shawkani explained al-Qaswah here to mean hardness of the heart to the extent that it does not accept admonition, fear punishment or show mercy to those deserving of it. [Tuhfah al-Dhakirin (p. 358)]
In this treatise Al-Hafiz Ibn Rajab al-Hanbali scholastically discusses the dangers of the hardness of the heart, its causes and how it can be treated in light of the Qur’an and the Sunnah.