Collection from riyaDh-U-SaliheeN

"Tell the believing men to lower their gaze (from looking at forbidden things)."[24:30]...

Abu Hurayrah r.a said:
The Prophet s.a.w 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 (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 & Muslim]