Genesis 6:4
3 So the Lord said, “My Spirit will not remain in humankind indefinitely, since they are mortal. They will remain for 120 more years.” 4 The Nephilim were on the earth in those days (and also after this) when the sons of God would sleep with the daughters of humankind, who gave birth to their children. They were the mighty heroes of old, the famous men. 5 But the Lord saw that the wickedness of humankind had become great on the earth. Every inclination of the thoughts of their minds was only evil all the time.
Jubilees 5:1
1 When mankind began to multiply on the surface of the entire earth and daughters were born to them, the angels of the Lord — in a certain (year) of this jubilee — saw that they were beautiful to look at. So they married of them whomever they chose. They gave birth to children for them and they were giants. 2 Wickedness increased on the earth. All animate beings corrupted their way — (everyone of them) from people to cattle, animals, birds, and everything that moves about on the ground. All of them corrupted their way and their prescribed course. They began to devour one another, and wickedness increased on the earth. Every thought of all mankind's knowledge was evil like this all the time.
Notes and References
"... In common with other Second Temple sources, (Some of the details in Jubilees 5 are strikingly reminiscent of the flood account in 1 Enoch 10-11) Jubilees assumes that the proximate cause of the food was the mating of the “sons of God” with the daughters of men in Genesis 6:1-4. These “sons of God” are, for Jubilees and other interpreters, the angels of the Lord. Te ofspring born of their union with humans were giants. The identification of these offspring, who are referred to as Nephilim in Genesis 6:4, as “giants” derives from an entirely separate biblical passage, Numbers 13:32-33, wherein some beings “of great stature” are also called Nephilim. Putting these two passages together, ancient interpreters understood the Nephilim in Genesis 6:4 likewise to be of great stature, the giants who resulted from the mating of angels with humans ..."
Kugel, James L. A Walk through Jubilees: Studies in the Book of Jubilees and the World of Its Creation (p. 52) Brill, 2012