Deuteronomy 3:16 (ESV)

16 and to the Reubenites and the Gadites I gave the territory from Gilead as far as the Valley of the Arnon, with the middle of the valley as a border, as far over as the river Jabbok, the border of the Ammonites;

The territory of the Reubenites and Gadites is now given in some detail. These tribes received the southern section of Gilead, with the Valley of the Arnon marking the southern limit, and the Jabbok forming the border between Gad and the half-tribe of Manasseh (to the north),