Genesis 6:1-4

And when men began to be multiplied on [the] earth, and had begat daughters, the sons of God saw the daughters of men that they were fair, and took wives to them of all which they had chosen. And God said, My spirit shall not dwell in man without end, for he is flesh; and the days of him shall be an hundred and twenty years. Soothly giants were on the earth in those days, forsooth after that the sons of God entered [in] to the daughters of men, and those daughters begat; these were mighty of the world and famous men.