On the authority of Abu ‘Abdullah an-Nu’man the son of Bashir (may Allah be pleased with them both), who said: I heard the Messenger of Allah (may the blessings and peace of Allah be upon him) say:
That which is lawful is plain and that which is unlawful is plain and between the two of 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 honor
(فَمَن اتَّقَى الشُّبُهَاتِ فَقَدْ اسْتَبْرَأَ لِدِيْنِهِ وَ عِرْضِهِ ),
but he who falls into doubtful matters falls into that which is unlawful, like the shepherd who pastures around a 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 it be whole, all the body is whole
(وَ إذَا صَلَحَتْ صَلَحَ الْجَسَدُ كُلُّهُ ),
and which, if it is diseased, all of it is diseased. Truly it is the heart.
It was related by al-Bukhari and Muslim; (recorded as Hadith # 6 in An-Nawawi’s “Forty Hadith”).