210.That which is lawful is plain and that which is unlawful is plain and between the two if them are doubtful matters about which not many people know. Thus he who avoids doubtful matters clears himself in regard to his religion and his honour, but he who falls into doubtful matters falls into that which is unlawful, like the shepherd who pastures around the sanctuary, all but grazing therein. Truly every king has a sanctuary, and truly Allah's sanctuary is His prohibitions. Truly in the body there is a morsel of flesh which, if diseased, all of it is diseased. Truly it is the heart.
(Bukhari, Muslim).
(Forty Hadith translated by. Ezzedin Ibrahim and Denys Johnson-Davies).