Numbers 32:25-29

And the children of Gad and the children of Reuben spake unto Moses, saying, Thy servants will do as my lord commandeth.

Our little ones, our wives, our flocks, and all our cattle, shall be there in the cities of Gilead:

But thy servants will pass over, every man armed for war, before the LORD to battle, as my lord saith.

So concerning them Moses commanded Eleazar the priest, and Joshua the son of Nun, and the chief fathers of the tribes of the children of Israel:

And Moses said unto them, If the children of Gad and the children of Reuben will pass with you over Jordan, every man armed to battle, before the LORD, and the land shall be subdued before you; then ye shall give them the land of Gilead for a possession: