Leviticus 10:1-7

Now Nadab and Abihu, the sons of Aaron, took their respective [ceremonial] censers, put fire in them, placed incense on it and offered strange (unauthorized, unacceptable) fire before the LORD, [an act] which He had not commanded them to do. And fire came out from the presence of the LORD and devoured them, and they died before the LORD. Then Moses said to Aaron, “This is what the LORD said: ‘I will be treated as holy by those who approach Me, And before all the people I will be honored.’ ” So Aaron, therefore, said nothing. Moses called Mishael and Elzaphan, the sons of Uzziel who was Aaron’s uncle, and said to them, “Come here, carry your relatives away from the front of the sanctuary and take them outside the camp.” So they came forward and carried them, still in their undertunics, outside the camp, as Moses had said. Then Moses said to Aaron and to his [younger] sons Eleazar and Ithamar, “Do not uncover your heads nor let your hair hang loose nor tear your clothes [as expressions of mourning], so that you will not die [also] and so that He will not express His wrath and anger toward all the congregation. But your relatives, the whole house of Israel, may mourn the burning which the LORD has brought about. You shall not even go out of the doorway of the Tent of Meeting, or you will die; for the LORD’S anointing oil is upon you.” So they did [everything] according to the word of Moses.