Flavius Josephus, Antiquities of the Jews
William Whiston, A.M., Ed.

[208] Hereupon an affliction befell Aaron, considered as a man and a father, but was undergone by him with true fortitude; for he had indeed a firmness of soul in such accidents, and he thought this calamity came upon him according to God's will: for whereas he had four sons, as I said before, the two elder of them, Nadab and Abihu, did not bring those sacrifices which Moses bade them bring, but which they used to offer formerly, and were burnt to death. Now when the fire rushed upon them, and began to burn them, nobody could quench it. Accordingly they died in this manner. And Moses bid their father and their brethren to take up their bodies, to carry them out of the camp, and to bury them magnificently. Now the multitude lamented them, and were deeply affected at this their death, which so unexpectedly befell them. But Moses entreated their brethren and their father not to be troubled for them, and to prefer the honor of God before their grief about them; for Aaron had already put on his sacred garments.