Genesis 6:1-4

Now it came about, when mankind 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 mankind were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, “My Spirit will not remain with man forever, because he is also 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 mankind, and they bore children to them. Those were the mighty men who were of old, men of renown.