Sahih al-Bukhari 52

Abu Nu'aym narrated to us. He said: Zakariyya narrated to us, from Amir, who said: I heard al-Nu'man ibn Bashir say: I 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 ambiguous matters that many people do not know. Whoever avoids ambiguous matters safeguards his religion and his honor; and whoever falls into doubtful matters is like a shepherd grazing around a protected pasture, liable soon to enter it. Indeed, every king has a protected pasture, and Allah's protected pasture on earth is what He has forbidden. Indeed, in the body there is a morsel of flesh: if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. It is the heart.’