Narrated An-Nu'man bin Bashir:
I heard Allah's Apostle, peace be upon him, 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 stray into it.
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.
That piece of flesh is the heart.