Says the God in surah Ghaafir verse 19 : "He knows the betrayal of eyes and and whatever hidden in the chest (heart)." It means that the God knows that our eyes are trying to see or watch something illegal while nobody sees us. The God knows whatever we think and have in our heart either the vision to commit evil or do charities.
Said the noble prophet Muhammad s.a.w : "Indeed gazing at parts of women (and men) which could stimulate is like poisonous arrows from the arrows of Devil as it is greatly destructive. Those who leave it (the act) behind due to the fear to the God the Most Highest will be granted the feeling of enjoyment in acts of worship and devotion to Him." The hadith is narrated by al-Hakim reported from Hudzaifa r.a. The source of the hadith is valid.
Said the noble prophet Muhammad s.a.w : "For every sons of Adam some parts of adultery. Eyes are committing adultery, and adultery happens when a person looks at things which he or she should not see. Hands are committing adultery, and adultery of hands happens when they touched or slapping something which is illegal. Legs are committing adultery when they walk to illegal places. Mouth committing adultery when it holds something which is illegal and 'heart and soul' is the place where all of illegal things related to adultery happens." Hadith is narrated by Abu Hurairah r.a and it explains that beginning adultery of genital is from the eyes as the portal to the illegal act.
Use your eyes only for necessary things such as when you do your work, your studies, and protect your body parts from stains of the world. Do things which is longed by those body parts. If you're doing a big work, then the responsibility to protect the parts of body is also big. Hands are longing to pluck fruits of the paradise and holding the drinks in the paradise. The eyes are longing to see the God from the paradise and it is not the same as seeing new or created "things." The grace of seeing the God appearance is incomparable to other graces in the world or in the hereafter. So, it is the responsibility of the body to protect the eyes and fulfilling their ambition for they are longing to see the God. Protect eyes with noble things for them to see Noble appearance and do not pollute them with obscene and dirty matters.