Zakaria bin Abi Za'idah زكريا بن أبي زائدة خالد · Commercial Transactions (Kitab Al-Buyu)

I heard the Messenger of Allah (ﷺ) say: What is lawful is clear and what is unlawful is clear, but between them are certain doubtful things. I give you an example for this. Allah has a preserve, and Allah's preserve is the things He has declared unlawful. He who pastures (his animals) round the preserve will soon fall into it. He who falls into doubtful things will soon be courageous.