Pirkei DeRabbi Eliezer
Rabbi Zadok said: From them were born the giants (Anakim), who walked with pride in their heart, and who stretched forth their hand to all (kinds of) robbery and violence, and shedding of blood, as it is said, "And there we saw the Nephilim, the sons of Anak" (Num. 13:33); and it says, "The Nephilim were on the earth in those days" (Gen. 6:4).