Genesis 6:2-4

the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. Then the LORD said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.” The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.