Musnad Ahmad 652:38

Sufyan narrated to us, he said: I memorized it from Abu Farwah first, then from Mujalid, who heard it from al-Sha'bi, who says: I heard al-Nu'man ibn Bashir saying: I heard the Messenger of Allah, peace and blessings of Allah be upon him. And whenever I heard him saying: "I heard the Messenger of Allah, peace and blessings of Allah be upon him," I would listen intently and draw near, fearing that I might not hear anyone else saying: "I heard the Messenger of Allah, peace and blessings of Allah be upon him." He says: "The lawful is clear, and the unlawful is clear, and between them are doubtful matters. Whoever leaves what is doubtful to him of sin, 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 the unlawful. And indeed, for every king there is a sanctuary, and the sanctuary of Allah on earth is His acts of disobedience," or he said: "His forbidden matters."