Midrasch zu Bereschit 1:21

(Gen. 1, 21) And God created the great sea monsters. Here in Babylon they translate this "sea gazelles." R. Jochanan, however, said: "It means leviathan— leviathan male and female, as it is written (Is. 27, 1) On that day will the Lord punish with His heavy and great and strong sword leviathan the flying serpent, and leviathan the crooked serpent, and He will slay the crocodile that is in the sea."

On the fifth day He brought forth from the water the Leviathan, the flying serpent, and its dwelling is in the lowest waters; and between its fins rests the middle bar of the earth. All the great sea monsters in the sea are the food for the Leviathan. Every day it opens its mouth, and the great sea monster destined to be eaten that day (tries) to escape and flee, but it enters the mouth of the Leviathan; and the Holy One, blessed be He, plays with it, as it is said, "This is the Leviathan, whom thou hast created to play with him" (Ps. 104:26).