Abud-Dardaa‘ (radiyallaahu ’anhu) said:
“For everything there is a polish and the polish for the heart is the dhikr of Allaah.” Imaam al-Bayhaqee relates from Ibn ’Umar (radiyallaahu ’anhu) that Allaah’s Messenger (sallallaahu ’alayhi wa sallam) said: “For everything there is a polish, and the polish for the hearts is the dhikr (remembrance) of Allaah. There is nothing more potent in saving a person from the punishment of Allaah than the dhikr of Allaah.” It was said: Not even Jihaad in the Path of Allaah. So he replied: “Not even if you were to continue striking with your sword until it breaks.”
[Saheeh: Related by Ahmad (4/352), from Mu’aadh Ibn Jabal (radiallaahu ’anhu). It was authenticated by al-Albaanee in Saheehul-Jaami’ (no. 5644).]
There is no doubt that hearts becomes rusty just as copper and silver coins become rusty. The polish for (this rust) is the dhikr of Allaah. This is because (dhikr) is like a polish (which polishes the heart) like a shiny mirror. When dhikr is left, the rust returns. But when dhikr resumes, then the heart is (again) polished. And hearts become rusty due to two things:- [i] neglecting remembering Allaah, and [ii] committing sins. The polish for these two things is:- [i] seeking Allaah’s forgiveness and [ii] dhikr.