Musnad Ahmad 652:22

Yahya ibn Sa'id narrated to us, from Mujalid, who said: 'Amir narrated to us, saying: I heard al-Nu'man ibn Bashir saying: I heard the Messenger of Allah (peace and blessings of Allah be upon him)—and he gestured with his two fingers toward his ears—saying: "Indeed, the lawful is clear, and the unlawful is clear, and between the lawful and the unlawful are doubtful matters. Many people do not know whether they are from the lawful or from the unlawful. Whoever leaves them, he seeks to clear himself for his religion and his honor. And whoever falls into them, he is about to fall into the unlawful. Just as the one who grazes around a sanctuary is about to graze within it. And for every king is a sanctuary, and indeed, the sanctuary of Allah is His prohibitions."