The Prophet (sal Allahu alaihi wa sallam) said: “The glance is a poisoned arrow of Shaitaan. Whoever lowers his gaze for Allah, He will bestow upon him a refreshing sweetness which he will find in his heart on the day that he meets Him.” [Ahmad, Al-Musnad]
The unrestrained glance results in the one who looks becoming attracted to what he sees, and in the imprinting of an image of what he sees in his heart. This can result in several kinds of corruption in the heart of the servant of Allah.
Shaitaan enters with the glance, for he travels with it, faster than the wind blowing through an empty place. He makes what is seen appear more beautiful than it really is, and transforms it into an idol for the heart to worship. Then he promises it false rewards, lights the fire of desires within it, and fuels it with the wood of forbidden actions, which the servant of Allah would not have committed had it not been for this distorted image.
It has been said that between the eye and the heart is an immediate connection; if the eyes are corrupted, then the heart follows. It becomes like a rubbish heap where all the dirt and filth and rottenness collect, and so there is no room for love for Allah, relating all matters to Him, awareness of being in His presence, and feeling joy at His proximity – only the opposite of these things can inhabit such a heart.