Musannaf Ibn Abi Shayba 9:1325

Jarir narrated to us, from Mansur, who said: Ibn Abbas said regarding [the verse]: "He knows the treachery of the eyes and what the breasts conceal." He said: A man is among a group of people, and a woman passes by them. He makes them see that he is lowering his gaze from her. But if he sees them being heedless, he looks at her. If he fears that they might notice him, he lowers his gaze from her, while Allah has already seen from his heart that he wished he could look at her private parts.