Nu’man bin Bashir reported: I heard Allah’s Messenger (peace and blessings of Allah be to him) as having said this (and Nu’man pointed towards his ears with his fingers): What is lawful is evident, and what is unlawful is evident, and in between them are the things doubtful which many people do not know. So, he who guards against doubtful things keeps his religion and honour blameless, and he who indulges in doubtful things indulges, in fact, in unlawful things, just as a shepherd who pastures his animals round a preserve will soon pasture them in it. Beware, every king has a preserve, and the things God has declared unlawful are His preserves. Beware, in the body there is a piece of flesh; if it is sound, the whole body is sound, and if it is corrupt, the whole body is corrupt, and listen it is the heart.
(Sahih Muslim)
Allah has defined the things which are lawful and those which aren’t. In between these lie certain things which are doubtful. The man who shuns doubtful things guards himself against pitfalls, since their use paves the way for falling into evil.
Initially no one dares commit evils; he starts from the acts which apparently have no evil around them, but the intention behind them is no good. He then proceeds steadily on this dangerous path without realising the enormity of danger lying ahead and ultimately falls victim to evil.
The Blessed Messenger has explained this process of moral deterioration with the help of a simile of a king’s preserve.