Genesis 6:1-4

And it happened that, when humankind began to multiply on the face of the ground, daughters were born to them. Then the sons of God saw the daughters of humankind, that they were beautiful. And they took for themselves wives from all that they chose. And Yahweh said, “My Spirit shall not abide with humankind forever in that he is also flesh. And his days shall be one hundred and twenty years.” The Nephilim were upon the earth in those days, and also afterward, when the sons of God went into the daughters of humankind, and they bore children to them.