Exodus 32:26-28 King James Version (KJV)
26 Then Moses stood in the gate of the camp, and said, Who is on the Lord's side? let him come unto me. And all the sons of Levi gathered themselves together unto him.
27 And he said unto them, Thus saith the Lord God of Israel, Put every man his sword by his side, and go in and out from gate to gate throughout the camp, and slay every man his brother, and every man his companion, and every man his neighbour.
28 And the children of Levi did according to the word of Moses: and there fell of the people that day about three thousand men.