Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 343
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 343, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אשת הבן, שנאמר "ערוות כלתך, לא תגלה" (ויקרא יח,טו).
Not to have intimate relations with one's son's wife, as [Leviticus 18:15] states: "Do not commit incest with your daughter-in-law."