Midrash su Levitico 20:12

10) (Vayikra 20:12) ("And if a man lies with his daughter-in-law, both of them shall be put to death. They have wrought corruption; their blood is in them.") "And a man if he lies with his daughter-in-law … they have wrought corruption": in that they have "twisted the cord" (of procreation, both father and son lying with the same woman). They shall be put to death": by stoning. You say it is by stoning, but perhaps it is by one of the other death penalties in the Torah! It is, therefore, written "their blood is in them," and, elsewhere (Vayikra 20:27) "their blood is in them." Just as the penalty there is stoning; here, too, it is stoning. We have heard the exhortation, but we have not heard the punishment; it is, therefore, written (Vayikra 18:15) "The nakedness of your daughter-in-law you shall not reveal." If "your daughter-in-law," I would think even a maidservant or a gentile woman; it is, therefore, written (Vayikra 18:15) "She is the wife of your son." Scripture speaks only of a woman with whom there is "wifehood" with your son — to exclude a maidservant and a gentile woman, where this does not obtain.