Leviticus 10:1-3

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.