On the authority of an-Nu’man ibn Basheer (may Allah be pleased with him), who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) say, “That which is lawful is clear and that which is unlawful is clear, and between the two of them are doubtful matters about which many people do not know. Thus he who avoids doubtful matters clears himself in regard to his religion and his honour, but he who falls into doubtful matters [eventually] falls into that which is unlawful, like the shepherd who pastures around a sanctuary, all but grazing therein. Truly every king has a sanctuary, and truly Allah’s sanctuary is His prohibitions. Truly in the body there is a morsel of flesh, which, if it be whole, all the body is whole, and which, if it is diseased, all of [the body] is diseased. Truly, it is the heart.”
Related by al-Bukhari and Muslim.
You can listen to the explanation of this hadith from the works and commentary of Shaykh Saleh Aalus Shaykh (hafidhahullah) which Ustadh Abu Iyaad (hafidhahullah) went through over two lessons some years ago. The second lesson also contains a brief commentary and explanation from the works of Shaykh Muhammad Salih ibn al-Uthaymeen (rahimahullah).