Numbers 32:28-30

28So Moses gave orders about them to the priest Eleazar, Joshua (son of Nun), and the family heads of the tribes of Israel.

29Moses told them, “If the tribes of Gad and Reuben get ready for battle in the Lord’s presence and cross the Jordan River with you and you conquer the land, give them Gilead as their own property.

30If they don’t get ready for battle and go with you, the land they will take possession of must be in Canaan with yours.”