Genesis 6:1-4

Now it happened, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were good in appearance; and they took wives for themselves, whomever they chose. Then Yahweh said, “My Spirit shall not strive with man forever because he indeed is flesh; nevertheless his days shall be 120 years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.