An-Nu'man b. Bashir reported God's Messenger as saying,“What is lawful is clear and what is unlawful is clear, but between them are certain doubtful things which many people do not recognize. He who guards against doubtful things keeps his religion and his honour blameless, but he who falls into doubtful things falls into what is unlawful, just as a shepherd who pastures his animals round a preserve will soon pasture them in it. Every king has a preserve, and God’s preserve is the things He has declared unlawful. In the body there is a piece of flesh, and the whole body is sound if it is sound, but the whole body is corrupt if it is corrupt. It is the heart.”(Bukhari and Muslim.)