THE FALL OF THE ANGELS "AND Adam lived an hundred and thirty years, and he begat in his own likeness after his image" (Gen. 5:3). Hence thou mayest learn that Cain was not of Adam's seed, nor after his likeness, nor after his image. (Adam did not beget in his own image) until Seth was born, who was after his father Adam's likeness and image, as it is said, "And he begat in his own likeness, after his image" (ibid.).
Cain — The Garden of Eden
Pirkei DeRabbi Eliezer 22:1