Adultery is prohibited by the seventh of the Ten Commandments and further addressed by Jesus in Matthew 5.
In Exodus 20:14:
14 Thou shalt not commit adultery.
In Matthew 5:27-28:
27 Ye have heard that it was said by them of old time, Thou shalt not commit adultery: 28 But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart.
In Proverbs 6:32:
32 But whoso committeth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul.