Al-Ḥāfiẓ Ibn Rajab [d. 795 AH] said: The saying of the Prophet [ﷺ]: ‘Truly there exists within the body a morsel of flesh; if it is healthy and pure the entire body follows it in health. If it is corrupt, then the entire body follows in corruption. Verily, [this morsel of flesh] is the heart.’ The [ḥadīth] proves that the righteousness of a slave’s physical actions, and his refraining from ḥarām and his avoidance of unclear matters all correlate directly with the purity of the actions of his heart.

So if his heart is pure, containing nought but the love of Allāh and love for that which is beloved to Allāh, [and he possesses] knowledgeable fear of Allāh and fear of falling into that which He detests, [he will find] himself engaging in righteous deeds only.

This will also cause him to completely abstain from that which is ḥarām and protect himself from that which is unclear [to him] for fear of falling into that which i [he knows] is ḥarām. If his heart harbours corruption through being completely conquered by following its desires and seeking what is beloved to it even if it is detestable to Allāh, he will find that his actions follow in that corruption. He presents himself before every disobedient act and unclear matter, insofar as he follows the desires of his [corrupt] heart.”

Source: Jāmiʿ al-ʿUlūm wa-al-Ḥikam: 218-219
Translated by: Riyāḍ al-Kanadī