Sayyiduna Nu'man bin Bashir (may Allah be pleased with him) says that he heard the Messenger of Allah (peace and blessings be upon him) say: "The lawful is clear and the unlawful is clear, and between them are doubtful matters which many people do not know. Whoever guards himself against the doubtful matters has safeguarded his honour and his religion, and whoever falls into the doubtful matters falls into the unlawful. It is like a shepherd who grazes (his animals) around a protected pasture: he is close to (his animals) entering it. Moreover, every king has a protected pasture; beware, the protected pasture of Allah is the things He has made unlawful. Beware, in the body there is a piece of flesh: when it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. Beware, it is the heart."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10400