Leviticus 10:1

1Now Nadab and Abihu, the sons of Aaron, took their respective [ceremonial] censers, put fire in them, placed incense on it and offered 10:1 What made the fire “strange” cannot be determined. They may have taken the fire from an unauthorized source, or used incense that did not meet the divine specifications. In any case, God’s laws and requirements are not subject to modification by man.strange (unauthorized, unacceptable) fire before the Lord, [an act] which He had not commanded them to do.