Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 331
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 331, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אשת אב, שנאמר "ערוות אשת אביך, לא תגלה" (ויקרא יח,ח).
Not to have intimate relations with one's father's wife, as [Leviticus 18:8] states: "Do not commit incest with your father's wife."