Rashi on Leviticus 10:5/רש"י על ויקרא י׳:ה׳

1בכתנתם IN THEIR GARMENTS — in those of the dead (not in their own garments). This tells us that their garments had not been destroyed by fire, but their souls alone (neither their clothing nor bodies showed signs of burning; they had been struck by fire inwardly): there entered into their nostrils something like two threads of fire which burnt them to death (Sifra, Shemini, Mechilta d'Miluim 2 34; Sanhedrin 52a).