Sahih Al Bukhari – Book of To Make The Heart Tender (Ar-Riqaq)
Volumn 008, Book 076, Hadith Number 481.
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Narated By Sahl bin Sa'd : Allah's Apostle said, "Whoever can guarantee (the chastity of) what is between his two jaw-bones and what is between his two legs (i.e. his tongue and his private parts), I guarantee Paradise for him."