"There is no doubt that the heart becomes covered with rust, just as metal dishes - silver, and their like - become rusty. So the rust of the heart is polished with dhikr (remembrance of Allah), for dhikr polishes the heart until it becomes like a shiny mirror. However, when dhikr is abandoned, the rust returns; and when it commences then the heart again begins to be cleansed. Thus the heart becoming rusty is due to two matters: sins and ghaflah (neglecting remembrance of Allah. Likewise, it is cleansed and polished by two things: istighfaar (seeking Allah's forgiveness) and dhikr." -
Ibn Al Qayyim