Exodus 32:26-29

26 then Moses stood in the gate of the camp, and said, "Whoever is on Yahweh's side, come to me!"  All the sons of Levi gathered themselves together to him.  27 He said to them, "Thus says Yahweh, the God of Israel, 'Every man put his sword on his thigh, and go back and forth from gate to gate throughout the camp, and every man kill his brother, and every man his companion, and every man his neighbor."  28 The sons of Levi did according to the word of Moses: and there fell of the people that day about three thousand men.  29 Moses said, "Consecrate yourselves today to Yahweh, yes, every man against his son, and against his brother; that he may bestow on you a blessing this day."