Numbers 31:3-7

And Moses spake unto the people, saying, Arm some of yourselves unto the war, and let them go against the Midianites, and avenge the LORD of Midian.

Of every tribe a thousand, throughout all the tribes of Israel, shall ye send to the war.

So there were delivered out of the thousands of Israel, a thousand of every tribe, twelve thousand armed for war.

And Moses sent them to the war, a thousand of every tribe, them and Phinehas the son of Eleazar the priest, to the war, with the holy instruments, and the trumpets to blow in his hand.

And they warred against the Midianites, as the LORD commanded Moses; and they slew all the males.