Diseases of the Hearts and Their Cures By Ibn Taymiyyah

Actions are distinguished, one from the other, with respect to their excellence in the Sight of Allah in accordance with the condition of the heart, not by their number or form, but rather due to the strength of the caller, his truthfulness, his sincerity and the extent to which he prefer Allah over himself. The heart has been singled out for this because it is the leader of the body, and through the purification of the leader the subjects become purified, and with his corruption they become corrupted. So if you, Observant of Allah, wish to cure your hear then it is upon you to be truthful with regards to seeking refuge with Allah and putting your trust in Him, to pray a great deal of supererogatory prayers, to perform the actions of obedience to Allah frequently, to pray the night prayer while the people are sleeping, and to treat your heart by making it continuously stick to the remembrances and by befriending only the righteous and to frequently recite the Quran. And Allah will indeed allow all of this to be preserved by him.