Zohar, Bereshit 45

1This sign was one of the twenty-two letters of the Torah, and God set it upon him to protect him.’ R. Judah said: ‘Cain rose up against Abel and killed him because he inherited his nature from the side of Samael, who brought death into the world. He was jealous of Abel on account of his female, as indicated by the words, “and it came to pass when they were in the field”, the word “field” signifying woman.’ On R. Hiya objecting that, according to the text, Cain was wroth because his offering was not accepted, R. Judah answered that this was a further reason.

2R. Judah further expounded the words, “If thou doest well, shall there not be an uplifting?” ‘The word “uplifting”,’ he said, ‘means the dignity which is due to a first-born, provided his actions warrant it.