You shall not commit adultery. (Exodus 20:14, Deuteronomy 5:18).
Can a man take fire in his bosom, and his clothes not be burned? Or can a man walk on hot coals, and his feet not be scorched? So is the one who goes in to his neighbor’s wife; whoever touches her will not go unpunished. (Proverbs 6:27-29).
You have heard that it was said, “You shall not commit adultery”; but I say to you, that everyone who looks on a woman to lust for her has committed adultery with her already in his heart. (Matthew 5:27-28).
And it was said, “Whoever sends his wife away, let him give her a certificate of divorce”; but I [Jesus] say to you that everyone who divorces his wife, except for the cause of unchastity, makes her commit adultery; and whoever marries a divorced woman commits adultery. (Matthew 5:31-32).
Let marriage be held in honor among all, and let the marriage bed be undefiled; for fornicators and adulterers God will judge. (Hebrews 13:4).
See also 1 Corinthians 6:9-10.