From Sahih Muslim (Hadith 5639):
Sahl b. Sa'd as-Sa'idi reported that a person peeped through the hole of the door of Allah's Messenger (ﷺ) and he had with him some pointed thing with which he had been adjusting (the hair of his head). Allah's Messenger (ﷺ) said to him:If I were to know that you had been peeping. I would have thrust it in your eyes. Allah has prescribed seeking permission because of protection against glance
Reference: Muslim 5639