Question:
If a person has with him hardness in his heart, then what are affairs that will soften the heart?

Answer:

There is nothing better or more necessary (for softening the heart) than the Noble Qur’aan. It is what will soften the hearts. Allah -the Exalted and Sublime – said:

‘’Those who believe (in the Oneness of Allah – Islamic Monotheism), and whose hearts find rest in the remembrance of Allah, Verily, in the remembrance of Allah do hearts find rest.’’ [Surah al-Rad Verse 28]

So the Qur’aan is what will soften the hearts, which if Allah were to reveal it on a mountain, you would surely have seen it humbling itself and rending asunder by the fear of Allah;

‘’Had We sent down this Quran on a mountain, you would surely have seen it humbling itself and rending asunder by the fear of Allah.’’ [Suratul Hashr Verse 21]

Likewise sitting with the righteous and listening to The Qur’aan, (listening) to admonitions and reminders–this is what will soften the hearts.

Source: http://alfawzan.af.org.sa/node/14944