I heard the Messenger of Allah (ﷺ) 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 honor, 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."

[Bukhari ; Muslim]