Sayyiduna Nu'man bin Bashir (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "The lawful is clear and the unlawful is clear, and between them are doubtful matters. Whoever leaves what is doubtful to him will all the more leave what is clear to him, and whoever ventures upon what he is in doubt about is close to falling into what is clear to him (as unlawful). Sins are the protected pasture of Allah, and whoever grazes (his animals) around the protected pasture is close to (their) entering it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10401