Rabbi Joshua ben Ḳorchah said: The Holy One, blessed be He, said: Heaven forbid! Never let the offerings of Cain and Abel be mixed up (with one another), even in the weaving of a garment, as it is said, "Thou shalt not wear a mingled stuff, wool and linen together" (Deut. 22:11). And even if it be combined let it not come upon thee, as it is said, "Neither shall there come upon thee a garment of two kinds of stuff mingled together" (Lev. 19:19).
Rabbi Zadok said: A great hatred entered Cain's heart against his brother Abel, because his offering had been accepted. Not only (on this account), but also because Abel's twin-sister was the most beautiful of women, and he desired her in his heart. Moreover he said: I will slay Abel my brother, and I will take his twin-sister from him, as it is said, "And it came to pass when they were in the field" (Gen. 4:8). "In the field" means woman, who is compared to a field.