The heart is healthy ONLY when it is functioning contrast to its desires. In other words, desires are the sickness of the heart and opposing them is the cure.
Ibn al-Qayyim, Sins & their Punishments
Al-Bayhaqee relates from Ibn ‘Umar radiallaahu ‘anhu that AlIaah’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)
Read More @ Polishing the Hearts by Imâm ibn al-Qayyim