Numbers 32:28-30

28Moses gave this message to Eleazar the priest, to Joshua the son of Nun and to the leaders of the family clans.

29He said to them, ‘The men in Reuben's tribe and in Gad's tribe must cross over the Jordan River to fight with you on behalf of the Lord. If they do that, you must give to them the land here in Gilead. When you have won power over the whole land, then this land will belong to them.

30If they refuse to fight together with the other tribes, they will have no land here. They must have part of the land in Canaan, like the other tribes.’