Hakeemul Ummat Hazrat Mawlana Ashraf Ali Thanawi ra said commenting on the verse 30 of surah Noor: (Tell the believing men to lower their gaze and be modest. That is purer for them. Lo! Allah is aware of what they do.) :

“Lowering one’s gaze is a major component of the teachings of sulook.

(That is, not seeing anything that Shariah commands impermissible to be seen).”

Masail us Sulook min Kalam Malik ul Mulook, page 375