Sahih Bukhari
To make the Heart Tender (Ar-Riqaq)
Volume: 008 / Book Number: 076 / Hadith Number: 481
Narrated 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."