Riyad as-Salihin Hadith 587

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].