Sahih Muslim
The Book Pertaining to Paradise, Its Description, Its Bounties and Its Intimates (Kitab Al-Jannat wa Sifat Naimiha wa Ahliha)
Book Number: 040 / Hadith Number: 6831
It is transmitted on the authority of Abu Huraira that Allah's Messenger (may peace be upon him) said: The molar tooth of an unbeliever or the canine teeth of an unbeliever will be like Ubud and the thickness of his skin a three night's journey.