Saying of Ibn Qayyim (RA):

“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 ahafah (neglecting remembrance of Allah). Likewise, it is cleansed and polished by two things: istighfaar (seeking Allah’s forgiveness) and Dhikr.”ibn Qayyim Ra