Catechism of the Catholic Church §2336 — excerpt
Jesus came to restore creation to the purity of its origins. ' But I say to you that every one who looks at a woman lustfully has already committed adultery with her in his heart."
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.