Musnad Ahmad 652:1

Hashim ibn al-Qasim narrated to us, Shayban narrated to us, from ‘Asim, from Khaythama and 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 the doubtful matters, he is more likely to leave the unlawful. The forbidden things of Allah are a sanctuary, and whoever grazes around the sanctuary is likely to graze within it."