Mālik ibn Dīnar (rahimahullāh) said:

“A servant is not afflicted with a punishment greater than hardness of the heart. “ (1)

Jāmi’ Bayān Al-Ilm wa Faḍlihi Vol.1 Page 565

See the following links for details on how hardness of the hearts occurs, and the means we can take to cure our hearts. BārakAllāhu Fīkum

• Shaykh Fawzan: Excessive Speech and No Remembrance of Allah
• Ibnul Qayyim: Purify the Heart
• Shaykh Rabī: Sign of a Hard Heart
• Shaykh Fawzān: What Softens the Heart
• Shaykh Uthaimīn: How to Soften the Heart