The lawful is clear and the unlawful is clear

On the authority of Abu Abdillah an-Nu'man ibn Bashir (may Allah be pleased with him and with his father), who heard the Messenger of Allah (may Allah bless him and grant him peace) say: "The lawful is clear and the unlawful is clear, and between them are doubtful matters about which most people do not know. Whoever guards against the doubtful protects his religion and his honour, and whoever falls into the doubtful will in the end fall into the unlawful, like a shepherd who pastures his flock around a sanctuary and risks that it will graze within it. Truly, every king has a sanctuary, and the sanctuary of Allah is what He has forbidden. And truly, in the body there is a morsel: if it is sound the whole body is sound, and if it is corrupt the whole body is corrupt — and it is the heart."

Related by al-Bukhari and Muslim.

[al-Bukhari 52; Muslim 1599]