Genesis 6:2-4

2the sons of God saw that the daughters of humankind were beautiful. Thus they took wives for themselves from any they chose.

3So the Lord said, “My spirit will not remain in humankind indefinitely, since they are mortal. They will remain for 120 more years.”

4The Nephilim were on the earth in those days (and also after this) when the sons of God were having sexual relations with the daughters of humankind, who gave birth to their children. They were the mighty heroes of old, the famous men.