Musnad Ahmad 652:72

Mu'ammal narrated to us, Sufyan narrated to us, from Abu Farwah, from al-Sha'bi, from al-Nu'man ibn Bashir, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The lawful is clear, and the unlawful is clear, and between them are doubtful matters. Whoever leaves what is doubtful to him of sin, or of the matter, then he is more likely to leave what is clear to him. And whoever dares to commit what he is in doubt about, he is likely to fall into what is clear. And whoever grazes around the sanctuary, he is likely to fall into it."