The Prophet Muhammad (S): When a believer sins a black dot appears on his heart. If he repents, pulls back and seeks forgiveness his heart is cleansed from the black dot, but if he increases his sins the dot grows larger, and this is the rust that Allah has mentioned in His Book: "No indeed! Rather their hearts have been sullied by what they have been earning." Rawḍat al-Wā'iẓīn