וְלַמִּזְרָ֗ח יָשַׁב֙ עַד־לְב֣וֹא מִדְבָּ֔רָה לְמִן־הַנָּהָ֖ר פְּרָ֑ת כִּ֧י מִקְנֵיהֶ֛ם רָב֖וּ בְּאֶ֥רֶץ גִּלְעָֽד׃
and eastward he dwelt even unto the entrance of the wilderness from the river Euphrates; because their cattle were multiplied in the land of Gilead.
Read midrashic interpretation of I Chronicles 5:9: and classical rabbinic exegesis.