Abu Hurairah (May Allah be pleased with him) said: The Prophet (PBUH) said, "Allah has written the very portion of Zina which a man will indulge in. There will be no escape from it. The Zina of the eye is the (lustful) look, the Zina of the ears is the listening (to voluptuous songs or talk), the Zina of the tongue is (the licentious) speech, the Zina of the hand is the (lustful) grip, the Zina of the feet is the walking (to the place where he intends to commit Zina), the heart yearns and desires and the private parts approve all that or disapprove it."[Al-Bukhari and Muslim].
This is the wording in Muslim; Al-Bukhari wording is a bit short.
Seeing, listening, walking, etc., are means of committing the sin of fornication and adultery, but they have been termed so metaphorically so that every Muslim saves himself from them. If he does not try to save himself from them, his desires will find support from the private parts of his body; that is, he will incline to immoral acts. If he keeps himself away from the means of fornication and adultery, his private parts will crush his desires, that is to say he will be saved from immoral acts. This is the reason it is said that one should avoid glances, smiles, salutation, conversation, promises and then meetings which occur by stages on one's way to fornication and adultery.