Negative commandment · Lo Ta’aseh #343 of 613
Not to have intimate relations with one's son's wife
שלא לגלות ערוות אשת הבן
The verse · Vayikra 18:15
עֶרְוַת כַּלָּתְךָ לֹא תְגַלֵּה אֵשֶׁת בִּנְךָ הִוא לֹא תְגַלֶּה עֶרְוָתָהּ
“Thou shalt not uncover the nakedness of thy daughter in law: she is thy son’s wife; thou shalt not uncover her nakedness.”
The Rambam’s words
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."
שלא לגלות ערוות אשת הבן, שנאמר "ערוות כלתך, לא תגלה" (ויקרא יח,טו).
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה