Genesis 6:1-4

And it came to pass, when men began to multiply on the face of the ground, and daughters were born unto them, that the sons of God saw the daughters of men that they were fair; and they took them wives of all that they chose. And the LORD said, My spirit shall not strive with man for ever, for that he also is flesh: yet shall his days be an hundred and twenty years. The Nephilim were in the earth in those days, and also after that, when the sons of God came in unto the daughters of men, and they bare children to them: the same were the mighty men which were of old, the men of renown.