Bereshis 6:1-4

And it came to pass, when HaAdam began to multiply on the face of ha'adamah, and banot were born unto them, That the bnei HaElohim saw the banot HaAdam that they were tovot; and they took them nashim of all which they chose. And HASHEM said, My Ruach [Hakodesh] shall not always strive with Adam, for that he also is basar: yet his yamim shall be a hundred and twenty shanah. HaNefilim were on ha'aretz in those yamim; and also after that, when the bnei HaElohim came in unto the banot HaAdam, and they bore children to them, the same became gibborim which were of old, men of renown.