(Sammael) riding on the serpent came to her, and she conceived; afterwards Adam came to her, and she conceived Abel, as it is said, "And Adam knew Eve his wife" (Gen. 4:1). What is the meaning of "knew"? (He knew) that she had conceived. And she saw his likeness that it was not of the earthly beings, but of the heavenly beings, and she prophesied || and said: "I have gotten a man with the Lord" (ibid.).
Rabbi Miasha said: Cain was born, and his wife, his twin-sister, with him. Rabbi Simeon said to him: Has it not already been said, "And if a man shall take his sister, his father's daughter, or his mother's daughter, and see her nakedness, and she see his nakedness; it is a shameful thing"? (Lev. 20:17). From these words know that there were no other women whom they could marry, and these were permitted to them, as it is said, "For I have said, The world shall be built up by love" (Ps. 89:2). With love was the world built up before the Torah had been given. Rabbi Joseph said: Cain and Abel were twins, as it is said, "And she conceived, and bare (with) Cain" (Gen. 4:1). At that hour she had an additional capacity for child-bearing (as it is said), "And she continued to bear his brother Abel" (Gen. 4:2).