Genesis 6:1-4

When human beings began to grow numerous on the earth and daughters were born to them, the sons of God saw how beautiful the daughters of human beings were, and so they took for their wives whomever they pleased. Then the LORD said: My spirit shall not remain in human beings forever, because they are only flesh. Their days shall comprise one hundred and twenty years. The Nephilim appeared on earth in those days, as well as later, after the sons of God had intercourse with the daughters of human beings, who bore them sons. They were the heroes of old, the men of renown. Warning of the Flood.