From Sahih Bukhari (Hadith 6612):
Narrated Ibn `Abbas:I did not see anything so resembling minor sins as what Abu Huraira said from the Prophet, who said, "Allah has written for the son of Adam his inevitable share of adultery whether he is aware of it or not: The adultery of the eye is the looking (at something which is sinful to look at), and the adultery of the tongue is to utter (what it is unlawful to utter), and the innerself wishes and longs for (adultery) and the private parts turn that into reality or refrain from submitting to the temptation
Reference: Bukhari 6612