Sahih Bukhari Volume 1, Book 2, Hadith 52

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 ‏"

Narrated By An-Nu'man bin Bashir : I heard Allah's Apostle 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.