The Diseases of the Hearts and their Cures by Shaykhul-Islaam Ibn Taymiyyah
Know O beloved reader that it is most important to spend one's time and energy in treating the heart, and hastening to correct and purify if from sickness and all sins. This is due to the heart occupying a great and lofty position in Islaam, for it is the place to which the Lord looks and the storehouse for tawheed, faith and sincerity.
Actions are distinguished, one from the other, with respect to their excellence in the Sight of Allaah in accordance with the condition of the heart, not by their number or form, but rather due to the strength of the caller, his truthfulnes, his sincerity and the extent to which he prefers Allaah over himself...
Al-Haafidh ibn Hajr al-Asqalaanee, may Allaah have mercy upon him and provide 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 Allaah, wish to cure your heart then it is upon you to be truthful with regards to seeking refuge with Allaah and putting your trust in Him, to pray a great deal of supererogatory prayers, to perform the actions of obedience to Allaah 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 Qur'aan. And Allaah will indeed allow all of this to be preserved by him."
Taken from the Compiler's Foreword