Sahih Bukhari Volume 003, Book 034, Hadith Number 267.
Narated By An-Nu'man bin Bashir : The Prophet said "Both legal and illegal things are obvious, and in between them are (suspicious) doubtful matters. So who-ever forsakes those doubtful things lest he may commit a sin, will definitely avoid what is clearly illegal; and who-ever indulges in these (suspicious) doubtful things bravely, is likely to commit what is clearly illegal. Sins are Allah's Hima (i.e. private pasture) and whoever pastures (his sheep) near it, is likely to get in it at any moment."