Numbers 31:1

Avenge the children of Israel of the Midianites: afterward shalt thou be gathered unto thy people.

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.