Numbers 32:24 (KJV)

For we will not inherit with them on yonder side Jordan, or forward; because our inheritance is fallen to us on this side Jordan eastward.

And Moses said unto them, If ye will do this thing, if ye will go armed before the LORD to war,

And will go all of you armed over Jordan before the LORD, until he hath driven out his enemies from before him,

And the land be subdued before the LORD: then afterward ye shall return, and be guiltless before the LORD, and before Israel; and this land shall be your possession before the LORD.

But if ye will not do so, behold, ye have sinned against the LORD: and be sure your sin will find you out.

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: