Midrasch zu Ijow 30:23

On the first day of the week he went into the waters of the upper Gihon until the waters reached up to his neck, and he fasted seven weeks of days, until his body became like a species of seaweed. Adam said before the Holy One, blessed be He: Sovereign of all worlds ! Remove, I pray Thee, my sins from me and accept my repentance, and all the generations will learn that repentance is a reality. What did the Holy One, blessed be He, do? He put forth His right hand, and accepted his repentance, and took away from him his sin, as it is said, "I acknowledge my sin unto thee, and mine iniquity have I not hid: I said, I will confess my transgressions unto the Lord; and thou forgavest the iniquity of my sin. Selah" (Ps. 32:5). Selah in this world and Selah in the world to come. Adam returned and meditated in his heart, and said: I know that death will remove me (to) "the house appointed for all living" (Job 30:28). Adam said: Whilst I am yet alive I will build for myself a mausoleum to rest therein. He planned and built for himself a mausoleum to rest therein beyond Mount Moriah. Adam said: If in the case of the tables (of stone), just because in the future they will be written by the finger (of God), the waters of the Jordan are destined to flee before them; how much more so will this be the case with my body which His two hands kneaded, and because He breathed into my nostrils the breath of the spirit of His mouth? After my death they will come and take my bones, and they will make them into an image for idolatry; but verily I will put || my coffin deep down beneath the cave and within the cave. Therefore it is called the Cave of Machpelah, which is a double cave. There Adam was put and his help-meet, Abraham and his help-meet, Isaac and his help-meet, Jacob and his help-meet. Therefore it is called "the city of four" (Kirjath Arba'); for four pairs (were buried there), and concerning them the verse says, "He entereth into peace; they rest in their beds, each one that walketh in his up-rightness" (Isa. 57:2).