Midrasch zu Tehillim 104:26

There are others who taught the saying of R. Isaac in the following connection: We are taught that R. Jose said: "In the future heathens will come to convert themselves." But do we then accept proselytes? Have we not taught that no proselytes will be accepted in the days of Messiah? So also was it in the days of David and Solomon that no proselytes were accepted. We must therefor say that the proselytes, however, attached themselves unto Israel, although they were not admitted, as R. Jose says that in the future idolaters will come and embrace Judaism and will lay Tephilin on their heads and arms, fringes on their dresses, m'zuzas on their doors. But as soon as they will see the war of Gog and Magog, and will question them: "With whom do you want to fight?" Whereto the answer will be: "With the Lord and his Messiah [as it reads (Ps. 2, 2) Against the Lord and his anointed], each of the nations will remove the above, and go away; and the Holy One will smile upon them. It is here that R. Isaac said "There is no smiling with the Lord, but on that day." But did not R. Juda say in the name of Rab: "There are twelve hours in a day, three hours of which the Holy One, praised be He! is occupied with the Torah. Three hours, He judges the whole world, and seeing that it is liable to be destroyed, He rises from the chair of judgment and sits down on the chair of mercy. The next three hours, He supports the whole world with food, from the very largest creature to the smallest one. And the last three hours, He plays with the leviathan, as it reads (Ps. 104, 26) Leviathan, whom Thou hast made to sport therein." Said R. Nachman b. Isaac: "With His creatures He smiles, but upon them He will smile only on that day."

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).