Exodus 32:27-28

And he said unto them, Thus saith Jehovah, the God of Israel, Put ye every man his sword upon his thigh, and go to and fro from gate to gate throughout the camp, and slay every man his brother, and every man his companion, and every man his neighbor. And the sons of Levi did according to the word of Moses: and there fell of the people that day about three thousand men.