Sahih Muslim Book 33, Hadith Number 6421.
Chapter : The measure of the son of Adam in regard to adultery, etc.
Abu Huraira reported Allah’s Apostle (may peace be upon him) as saying: Verily Allah has fixed the very portion of adultery which a man will indulge in, and which he of necessity must commit. The adultery of the eye is the lustful look, and the adultery of the tongue is the licentious speech, the heart desires and yearns, which the parts may or may not put into effect.