Hadith 6, 40 Hadith an-Nawawi
On the authority of an-Nu’man ibn Basheer (ra), who said:
I heard the Messenger of Allah (ﷺ) say, “That which is lawful is clear and that which is unlawful is clear, and between the two of them are doubtful matters about which many people do not know. Thus he who avoids doubtful matters clears himself in regard to his religion and his honor, but he who falls into doubtful matters [eventually] 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, and which, if it is diseased, all of [the body] is diseased. Truly, it is the heart.”

[ Bukhari & Muslim ]
عَنْ أَبِي عَبْدِ اللَّهِ النُّعْمَانِ بْنِ بَشِيرٍ رَضِيَ اللَّهُ عَنْهُمَا، قَالَ: سَمِعْت رَسُولَ اللَّهِ صلى الله عليه و سلم يَقُولُ: "إنَّ الْحَلَالَ بَيِّنٌ، وَإِنَّ الْحَرَامَ بَيِّنٌ، وَبَيْنَهُمَا أُمُورٌ مُشْتَبِهَاتٌ لَا يَعْلَمُهُنَّ كَثِيرٌ مِنْ النَّاسِ، فَمَنْ اتَّقَى الشُّبُهَاتِ فَقْد اسْتَبْرَأَ لِدِينِهِ وَعِرْضِهِ، وَمَنْ وَقَعَ فِي الشُّبُهَاتِ وَقَعَ فِي الْحَرَامِ، كَالرَّاعِي يَرْعَى حَوْلَ الْحِمَى يُوشِكُ أَنْ يَرْتَعَ فِيهِ، أَلَا وَإِنَّ لِكُلِّ مَلِكٍ حِمًى، أَلَا وَإِنَّ حِمَى اللَّهِ مَحَارِمُهُ، أَلَا وَإِنَّ فِي الْجَسَدِ مُضْغَةً إذَا صَلَحَتْ صَلَحَ الْجَسَدُ كُلُّهُ، وَإذَا فَسَدَتْ فَسَدَ الْجَسَدُ كُلُّهُ، أَلَا وَهِيَ الْقَلْبُ". 
[رَوَاهُ الْبُخَارِيُّ]، [وَمُسْلِمٌ]
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