Sahih Muslim Book 1, Hadith Number 109.

Chapter : Concerning diminution of Belief due to sins and its separation from the sinner; negation of the excellence of Islam at the time of committing sin.

It is narrated on the authority of Abu Huraira: A fornicator who fornicates is not a believer as long as he commits fornication, and no one who steals is a believer as long as he commits theft, and no one who drinks wine is a believer as long as he drinks it, and repentance may be accepted after that.