Imam Sadiq (as) on Lowering the Gaze

On Lowering the Gaze
There is nothing more gainful than lowering one's gaze, for the sight is
not lowered from things, which Allah has forbidden unless the witnessing
of majesty and glory has already come to the heart.
The Commander of the Faithful was asked what could help in lowering
one's gaze. He said, 'Submission to the power of Him Who is aware
of your secret. The eye is the spy of the hearts and the messenger of the
intellect; therefore lower your gaze from whatever is not appropriate to
your faith, from whatever your heart dislikes and from whatever your
intellect finds repugnant.'
The Holy Prophet said, 'Lower your eyes and you will see wonders.'
Allah said,

Say to the believing men that they cast down their looks and guard
their private parts. (24:30)

‘Isa [a] said to the disciples, 'Beware of looking at forbidden things, for
that is the seed of desire and leads to deviant behavior.'
Yahya [a] said, 'I would prefer death to a glance which is unnecessary.'
'Abdallah ibn Mas'ud said to a man who had visited a woman while
she was ill, 'It would have been better for you to lose your eyes than to
have visited your sick person.'
Whenever the eye looks at something forbidden, a knot of desire is
tied in the person's heart, and that knot will only be untied by one of two
conditions: either by weeping out of grief and regret in true repentance,

or by taking possession of what one desired and looked at. And if a person
takes possession unjustly, without repentance, then that will take
him to the Fire.
As for the one who repents of it with grief and regret, his abode is the
Garden and his destiny is Allah's favor.

[From the book: Lantern of Path, Chapter 4]