The following is an excerpt from ‘Diseases of the Heart and their Cures’ by Ibn Taymiyyah:
It is most important to spend one’s time and energy in treating the heart, and hastening to correct and purify it from sickness and all sins. This is due to the heart occupying a great and lofty position in Islam, for it is the place to which the Lord looks and the storehouse for tawhid, faith, and sincerity.
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 or her truthfulness, his or her sincerity and the extent to which he or she prefers Allah over himself or herself.
The heart forms the foundation, it is the owner of the limbs, and the limbs are its soldiers, so when the owner becomes purified its soldiers become purified, and when the owner becomes sullied then its soldiers become sullied.
Al-Hafidh ibn Hajr al-Asqalani, may Allah have mercy upon him and nourish us with his knowledge, said: ‘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, O servant of Allah, wish to cure your heart 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 superogatory 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 ma king it continuously stick to the remembrances and by befriending only the righteous … and to frequently recite the Qur’an. May Allah allow all of this to be preserved by him.’