Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 347
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 347, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אשת איש, שנאמר "ואל אשת, עמיתך--לא תיתן שכובתך" (ויקרא יח,כ).
Not to have intimate relations with a married woman, as [Leviticus 18:20] states: "Do not lie carnally with your neighbor's wife."