The truthful and sound heart is that which is secure from every carnal desire that opposes the order and prohibition of Allaah. It is secure from every doubt and uncertainty that would obscure or go against His narrative. It is secure from displaying servitude to any other than Him; just as it is secure from seeking ruling from any other than His Messenger sallallaahu `alayhi wasallam. Therefore it becomes sound through loving Allaah and seeking the ruling of His Messenger. It becomes sound through showing Him fear, hope, trust and reliance, penitence, and humility; it prefers what pleases Him in every circumstance and distances itself from everything that would displease Him in every possible way. This is the reality of servitude (`uboodiyyah) which can only be directed to Allaah alone.
[The Key to Paradise, Ibn Rajab al-Hanbali]