The importance of this topic is highlighted by the supplication of the Prophet (ﷺ) 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 ash-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)]

Malik b. Dinar said: “A slave has never been punished with anything more severe than hardness of the heart.”

In this treatise Al-Hafidh 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.