Sunan Ibn Majah, Chapter: The Chapters on Tribulations

While on the pulpit, pointing with this fingers towards his ears, Nu'man bin Bashir said: While on the pulpit, pointing with this fingers towards his ears, Nu'man bin Bashir said:"I heard the Messenger of Allah (saw) say: 'That which is lawful is plain and that which is unlawful is plain, and between them are matters that are not clear, about which not many people know. Thus he who guards against the unclear matters, he clears himself with regard to his religion and his honor. But he who falls into the unclear matters, he falls into that which is unlawful. Like the shepherd who pastures around a sanctuary, all but grazing therein. Every king has a sanctuary. And beware! Allah's sanctuary is His prohibitions. Beware! In the body there is a piece of flesh which, if it is sound, the whole body will be sound, and if it is corrupt, the whole body will be corrupt. It is the heart.'"