Musar zu Wajikra 20:14

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.