Imam ‘Abdul ‘Azeez Ibn Baaz (may Allaah have mercy upon him and grant him Paradise)
The Shaykh was asked: How does a person escape the hardening of the heart and what are the causes of this?
Answer: The things that cause the hardening of the heart are sins and disobedience, frequent heedlessness [i.e. of the remembrance of Allaah] and accompanying the heedless and immoral sinners. All of these corruptive [behaviours] cause the hardening of the heart.
As for the softening, purity and tranquillity of the heart, then this is achieved through the obedience of Allaah (the Mighty and Majestic), accompanying the righteous and preserving ones time with the remembrance of Allaah, recitation of the Quran and seeking forgiveness.
Whosoever preserves his time by [utilising it] for the remembrance of Allaah, recitation of the Quran, and accompanying the righteous and staying away from the heedless and wicked, then verily his heart will be healthy and soft.
Allaah, The Most High Said:
“Verily, in the remembrance of Allah do the hearts find rest.” [Surat Ar-Ra`d 13:28]