Allah (s.w.t.) said: "Tell the believing men to cast down their looks and guard their private parts, that is purer for them, surely Allah is well acquainted with what they do."
Sura An-Nur, No. 24, verse 30
Imam Sadiq [a] said: "An unlawful look (at na-mahram) is an arrow from the arrows of Satan which is poisonous. He who abandons it for the sake of Allah, Almighty and Glorious, and not for something else, consequently, Allah will give him a Faith that he finds its pleasure (in his self)."
Man La Yahduruhul Faqih, vol. 4, p. 18
Imam Sadiq [a] said: "The fornication of the eyes is the intercourse of unlawful (lustful) looks, the fornication of the lips is a forbidden kiss (of na-mahram), and the fornication of the hands is touching (hands or other limbs of a na-mahram), whether the response of the genital affirm it or disaffirms it ."
Al-Kafi, vol. 5, p. 559
Imam Baqir [a] said: "The Holy Prophet [s] cursed the man who looks at the private part of a woman who is not lawful to him, and also the man proves treacherous to his religious brother's wife, and also the man that people need his help (because of his position) but he asks for a bribe from them."
AI-Kafi, vol. 5, p. 559