The Means of Blocking The Paths to Fornication – By Shaikh Saaleh Al-Fawzaan (may Allaah preserve him)
In The Name of Allaah, The Most Merciful, The Bestower of Mercy
Firstly: Allaah commanded (us) to lower the gaze because the eyes are a means among the means to fornication. [Allaah said: قُلْ لِلْمُؤْمِنِينَ يَغُضُّوا مِنْ أَبْصَارِهِمْ وَيَحْفَظُوا فُرُوجَهُمْ – Tell the believing men to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts, etc.) 24:30]. Therefore, lowering the gaze is a means to protecting the private parts. And Allaah (جلال وعلا) said: [يَعْلَمُ خَائِنَةَ الْأَعْيُنِ وَمَا تُخْفِي الصُّدُورُ – Allah knows the fraud of the eyes, and all that the breasts conceal (40:19)]
What is fraud of the eyes? It is when one steals a glance at what Allaah has forbidden. When (a person) is with the people and the people are looking at him, he lowers his gaze; but when he sees that they are inattentive, he steals a glance at the unlawful. This is the fraud of the eyes and Allaah knows about it.
[Slightly paraphrased. Source: It’haaf At-Tullaab Bi-Sharhi Mandhoomah Al-Aadaab’ page 89]