Ibnul Qayyim (rahimahullah):
If your heart is distressed and troubled, then (know) there is nothing that will calm it other than the remembrance of Allah.
Madaarij as-Saalikeen v.2 pg.480
Shaykh Uthaymīn was asked:
What are the signs of a responsive heart? And the signs of an unresponsive heart?
Shaykh Uthaymīn:
A sign of a responsive heart, is that it humbles itself to the remembrance of Allāh.
And the sign of an unresponsive heart is that it is not affected by what it hears, from the verses of Allah.
Fatāwa Su’āl ‘ala al-Hātif Vol.2 pg 596-597
إِلَّا مَنْ أَتَى اللَّهَ بِقَلْبٍ سَلِيمٍ
Except him who brings to Allāh a clean heart [clean from Shirk (polytheism) and Nifāq (hypocrisy)]. Ash-Shu’ara:89
Shaykh Rabī ibn Hādi Al-Madkhalī (hafidahullāh):
Ponder, is your heart sound?
The sound heart is that which fights Shirk and rejects falsehood, it accepts the truth and refutes falsehood with all its might, this is the sound heart. So, if you notice that you accept the truth from whoever brings it to you (you accept the truth; the truth guides you and your goal is the truth) then by Allāh your heart and intellect is sound.
And, if you realise that you reject the truth when it comes to you from a certain direction, and you accept falsehood when it comes from another direction and you behave as if it is the truth; then know that your heart is sick.
Majmu’ Ar- Rā’iq Pgs. 485-486
“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
[Saheeh Al-Bukhaari Vol 8. Hadeeth Number 6507. The Book of Ar-Riqaq (Softening the Hearts)