﴾يَعْلَمُ خَائِنَةَ الْأَعْيُنِ وَمَا تُخْفِي الصُّدُورُ﴿
Translation: He knows the deceptions of the eyes, and what the hearts conceal.
Ibn ‘Abbas: A man is with a group of people when a woman passes.
He shows them that he’s lowered his gaze from looking at her.
But when he sees they’re not paying attention, he looks at her.
If he fears they’ll notice, he lowers his gaze, but Allaah has seen in his heart that he wishes he could see her body (private areas)!
Source: Al-Zuhd li Hanad bin al-Sari (no. 1428)