Genesis 6:1 – 4
New International Version (NIV)
Wickedness in the World
1 When human beings began to increase in number on the earth and daughters were born to them, 2 the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. 3 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.”
4 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.
From BibleGateway.com: http://www.biblegateway.com/passage/?search=Genesis 6:1-4&version=NIV