Deuteronomy 3:18-19

At that time I told the men of Reuben, Gad, and East Manasseh: The LORD our God told me to give you this land with its towns, and that's what I have done. Now your wives and children can stay here with your large flocks of sheep and goats and your large herds of cattle. But all of you men that can serve in our army must cross the Jordan River and help the other tribes, because they are your relatives.