Chapter: The prohibition of gazing at a non-mahram woman or an attractive beardless youth when there is no legitimate sharʿī need to do so.

Allah the Exalted says, “Tell the believing men to lower their gaze.” (Q 24:30)
He also says, “Indeed the hearing, the sight and the heart—all of these a person will be questioned about.” (Q 17:36)
And He says, “He knows the furtive glance of the eyes and what the chests conceal.” (Q 40:19)
And He says, “Truly, your Lord is ever on the watch.” (Q 89:14)

[Explanation]
Imām al-Nawawī, may Allah have mercy on him, writes in Riyāḍ al-Ṣāliḥīn: “Chapter on the prohibition of looking at a non-mahram woman and at a handsome beardless youth when there is no need sanctioned by the Sacred Law.”

A non-mahram woman is any woman with whom you have no relationship of maḥram-ship, whether she is a close relative or a distant one.
A beardless youth (amrad) is a young man whose beard has not yet appeared and whose moustache is not yet thick—that is, his moustache is still faint and no beard has grown.
“Handsome” is the opposite of “ugly.”

Gazing at a non-mahram woman is forbidden, as the author—may Allah have mercy on him—states, because Allah has commanded the lowering of the gaze: “Tell the believing men to lower their gaze and guard their private parts; that is purer for them. Allah is well aware of what they do.” (Q 24:30) Thus Allah commands two things: lowering the gaze and safeguarding chastity. This shows that failing to lower the gaze is a cause for failing to safeguard one’s private parts. When a person allows his eyes to wander, his heart becomes attached to women; the gaze persists until he draws near to the woman, speaks to her, converses with her, makes promises—and then the act of fornication occurs.