Doubtless, a heart grows tarnished like brass or silver, and its polish is by remembrance, which may make it shine like a crystal mirror.  So when one neglects the remembrance it tarnishes, and when he returns it shines.  Since the heart is tarnished by two things- heedlessness and sin- it is polished by two things: remembrance and asking forgiveness.

—Ibn al-Qayyim [d. 751H/1350CE]

(Read on pg 47, Ibn Qayyim al-Jawziyya, The Invocation of God. The Islamic Texts Society. UK:2000 .)