Numbers 32:26 (KJV)

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.

Build you cities for your little ones, and folds for your sheep; and do that which hath proceeded out of your mouth.

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

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:

But if they will not pass over with you armed, they shall have possessions among you in the land of Canaan.

And the children of Gad and the children of Reuben answered, saying, As the LORD hath said unto thy servants, so will we do.