Numbers 36:3

And if they be married to any of the sons of the other tribes of the children of Israel, then shall their inheritance, according to the provision, as it now stood, be taken from the inheritance of our fathers and shall be put to the inheritance of the tribe whereunto they are received; so shall it be taken from the lot of our inheritance. The consequence, as they saw it, would be that the territory of the several tribes would not remain intact and that a general confusion would result.