Midrasch zu Ijow 40:15

On the sixth day He brought forth from the earth a beast (Behemoth) which lies stretched out on a thousand hills and every day has its pasture on a thousand hills, and overnight (the verdure) grows of its own account as though he had not touched it, as it is said, "Surely the mountains bring him forth food" (Job 40:20). The waters of the Jordan give him water to drink, for the waters of the Jordan surround all the earth, half thereof (flow) above the earth and the other half below the earth, as it is said, "He is confident, though Jordan swell even to his mouth" (Job 40:23). This (creature) is destined for the day of sacrifice, for the great banquet of the righteous, as it is said, "He only that made him can make his sword to approach unto him" (Job 40:19).