Musannaf Ibn Abi Shayba 13:2044

Abu Bakr narrated to us, he said: Zakariya narrated to us, from Amir, he said: I heard al-Nu'man ibn Bashir delivering a sermon, and he was pointing with his fingers toward his ears, saying: I heard the Prophet, may Allah bless him and grant him peace, saying: "The lawful is clear and the unlawful is clear, and between them are doubtful matters. Whoever avoids the doubtful matters has sought purity for his religion and his honor, and whoever falls into the doubtful matters falls into the unlawful, like the shepherd who grazes around a sanctuary; he is soon to graze within it. Know that every king has a sanctuary, and the sanctuary of Allah is His prohibitions. Know that in the body 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. Know that it is the heart."