Imam Ja'far al-Sadiq (a.s.) says:
"A young man from the Ansar met a woman is Madina. At that time, women used to wear a veil, so he looked at her while she was coming toward him. Men she passed by he continued looking after her until he entered one of the alleys. He continued looking as he passed along the alley until a bone on a wall slashed his face and the woman passed out of sight. Then, unexpectedly, he saw blood flowing over his clothes and chest. He said, 'I must go and inform the Messenger of Allah (s.a.w.) about the incident.' When the Messenger of Allah (s.a.w.) saw him in this condition he said, 'What happened?' When he informed the Messenger of Allah (s.a.w.) about the event in details, the angel Gabriel (a.s.) came down with this verse:
'Say to the believing men that they should lower their gaze and guard their private parts. That is purer for them. Truly Allah is Cognizan of what they do.' Holy Qur'an (24:30).