Sahih al-Bukhari 52
Narrated An-Nu'man bin Bashir:
I heard Allah's Messenger (ﷺ) saying, 'Both legal and illegal things are 
     evident but in between them there are doubtful (suspicious) things and
     most of the people have no knowledge about them. So whoever saves 
     himself from these suspicious things saves his religion and his honor.
     And whoever indulges in these suspicious things is like a shepherd who
     grazes (his animals) near the Hima (private pasture) of someone else 
     and at any moment he is liable to get in it. (O people!) Beware! Every
     king has a Hima and the Hima of Allah on the earth is His illegal 
     (forbidden) things. Beware! There is a piece of flesh in the body if 
     it becomes good (reformed) the whole body becomes good but if it gets 
     spoilt the whole body gets spoilt and that is the heart.
حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا زَكَرِيَّاءُ، عَنْ عَامِرٍ، قَالَ سَمِعْتُ النُّعْمَانَ بْنَ بَشِيرٍ، يَقُولُ سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏
"‏ الْحَلاَلُ بَيِّنٌ وَالْحَرَامُ بَيِّنٌ، وَبَيْنَهُمَا مُشَبَّهَاتٌ لاَ يَعْلَمُهَا كَثِيرٌ مِنَ النَّاسِ، فَمَنِ اتَّقَى الْمُشَبَّهَاتِ اسْتَبْرَأَ لِدِيِنِهِ وَعِرْضِهِ، وَمَنْ وَقَعَ فِي الشُّبُهَاتِ كَرَاعٍ يَرْعَى حَوْلَ الْحِمَى، يُوشِكُ أَنْ يُوَاقِعَهُ‏.‏ أَلاَ وَإِنَّ لِكُلِّ مَلِكٍ حِمًى، أَلاَ إِنَّ حِمَى اللَّهِ فِي أَرْضِهِ مَحَارِمُهُ، أَلاَ وَإِنَّ فِي الْجَسَدِ مُضْغَةً إِذَا صَلَحَتْ صَلَحَ الْجَسَدُ كُلُّهُ، وَإِذَا فَسَدَتْ فَسَدَ الْجَسَدُ كُلُّهُ‏.‏ أَلاَ وَهِيَ الْقَلْبُ ‏"‏‏.‏