You shall not uncover the nakedness of your daughter-in-law--she is your son's wife--you shall not uncover her nakedness.
Leviticus 18:15
Surrounding context
18:14 You shall not uncover the nakedness of your father's brother. You shall not approach his wife; she is your aunt.
18:15 You shall not uncover the nakedness of your daughter-in-law--she is your son's wife--you shall not uncover her nakedness.
18:16 You shall not uncover the nakedness of your brother's wife; it is your brother's nakedness.