يَعْلَمُ خَآئِنَةَ ٱلْأَعْيُنِ وَمَا تُخْفِى ٱلصُّدُورُ
Allah knows the fraud of the eyes, and all that the breasts conceal [Quran, Ghāfir (40):19]
It is reported that ‘Abdullāh b. ‘Abbās – Allāh be pleased with him – said, explaining this verse:
A man is with a group of people when a woman passes. He shows them that he has lowered his gaze from looking at her. But when he sees they are not paying attention, he looks at her. If he fears they will notice, he lowers his gaze, but Allāh has seen in his heart that he wishes he could see her body (private areas).
Hunād b. Al-Sarī, Al-Zuhd article 1428.