كتاب المقدمات
1
The Book of Miscellany
An-Nu'man bin Bashir (May Allah be pleased with them) reported:
Messenger of Allah (ﷺ) said, "What is lawful is clear and what is unlawful is clear, but between them are certain doubtful things which many people do not know. So he who guards against doubtful things keeps his religion and his honour blameless. But he who falls into doubtful things falls into that which is unlawful, just as a shepherd who grazes his cattle in the vicinity of a pasture declared prohibited (by the king); he is likely to stray into the pasture. Mind you, every king has a protected pasture and Allah's involved limits is that which He has declared unlawful. Verily, there is a piece of flesh in the body, if it is healthy, the whole body is healthy, and if it is corrupt, the whole body is corrupt. Verily, it is the heart."

[Al-Bukhari and Muslim].

وعن النعمان بن بشير رضي الله عنهما قال‏:‏ سمعت رسول الله صلى الله عليه وسلم يقول‏:‏ “إن الحلال بين، وإن الحرام بين، وبينهما مشتبهات لا يعلمهن كثير من الناس، فمن اتقى الشبهات، استبرأ لدينه وعرضه، ومن وقع فى الشبهات، وقع فى الحرام، كالراعى يرعى حول الحمى يوشك أن يرتع فيه ألا وإن لكل ملك حمى، ألا وإن حمى الله محارمه، ألا وإن فى الجسد مضغة إذا صلحت صلح الجسد كله، وإذا فسدت فسد الجسد كله‏:‏ ألا وهى القلب” ‏(‏‏(‏متفق عليه‏.‏ وروياه من طرق بألفاظ متقاربة‏)‏‏)‏
Arabic/English book reference : Book 1, Hadith 588