Musnad Ahmad 652:28

And I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "Indeed, the lawful is clear, and the unlawful is clear, and between them are doubtful matters which many people do not know. So whoever avoids the doubtful matters, he seeks to clear himself for his religion and his honor. And whoever falls into them, falls into the unlawful, like the shepherd who grazes around a sanctuary, he is soon to graze within it. "Behold, every king has a sanctuary, and the sanctuary of Allah is what He has made unlawful. Behold, in the human being there is a piece of flesh; if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt. Behold, it is the heart."