וְאִ֗ישׁ אֲשֶׁ֨ר יִקַּ֧ח אֶת־אִשָּׁ֛ה וְאֶת־אִמָּ֖הּ זִמָּ֣ה הִ֑וא בָּאֵ֞שׁ יִשְׂרְפ֤וּ אֹתוֹ֙ וְאֶתְהֶ֔ן וְלֹא־תִהְיֶ֥ה זִמָּ֖ה בְּתוֹכְכֶֽם׃
Seorang laki-laki yang memperistri seorang perempuan beserta ibunya, itu adalah perbuatan mesum; ia dan mereka harus dibakar dengan api, supaya jangan ada perbuatan mesum di tengah-tengah kamu.
Shenei Luchot HaBerit
באש ישרופו אותו ואתהן . When someone marries a wife [mother-in-law. Ed.] and her mother (20,14), the Torah decrees death by "burning" for all three parties. Our sages teach us the type of "burning" the Torah has in mind for this transgression as well as for all other instances where death by "burning" is decreed. It is similar to what happened to Nadav and Avihu whose souls were burned while their bodies remained lifeless but intact. If the Torah would have written ישרפו באש, the fire would have first consumed the body as is the case when someone is "burned at the stake" Since the Torah mentions the word אש first, it is clear that a different kind of burning is meant.