1 Enoch 6:2
1 And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. 2 And the angels, the children of the heaven, saw and lusted after them, and said to one another: 'Come, let us choose us wives from among the children of men and beget us children.' 3 And Semjâzâ, who was their leader, said unto them: 'I fear ye will not indeed agree to do this deed, and I alone shall have to pay the penalty of a great sin.' 4 And they all answered him and said: 'Let us all swear an oath, and all bind ourselves by mutual imprecations not to abandon this plan but to do this thing.' 5 Then sware they all together and bound themselves by mutual imprecations upon it.
Pirkei DeRabbi Eliezer 22:4
Rabbi Simeon said: From Seth arose and were descended all the generations of the righteous. From Cain arose and were descended all the generations of the wicked, who rebel and sin, who rebelled against their Rock, and they said: We do not need the drops of Thy rain, neither to walk in Thy ways, as it is said, "Yet they said unto God, Depart from us" (Job 21:14). Rabbi Meir said: || The generations of Cain went about stark naked, men and women, just like the beasts, and they defiled themselves with all kinds of immorality, a man with his mother or his daughter, or the wife of his brother, or the wife of his neighbour, in public and in the streets, with evil inclination which is in the thought of their heart, as it is said, "And the Lord saw that the wickedness of man was great in the earth" (Gen. 6:5). Rabbi said: The angels who fell from their holy place in heaven saw the daughters of the generations of Cain walking about naked, with their eyes painted like harlots, and they went astray after them, and took wives from amongst them, as it is said, "And the sons of Elohim saw the daughters of men that they were fair; and they took them wives of all that they chose" (Gen. 6:2). Rabbi Joshua said: The angels are flaming fire, as it is said, "His servants are a flaming fire" (Ps. 104:4), and fire came with the coition of flesh and blood, but did not burn the body; but when they fell from heaven, from their holy place, their strength and stature (became) like that of the sons of men, and their frame was (made of) clods of dust, as it is said, "My flesh is clothed with worms and clods of dust" (Job 7:5).
Notes and References
"... In suggesting the continued transmission of the angelic descent myth in Rabbinic circles, scholars sometimes point to the latest of these translations, Targum Pseudo-Jonathan. This targum renders Nephilim in Gen 6:4 with the gloss 'Semhazai and Azael, these fell from heaven.' Alexander, for instance, cites this allusion to the angelic descent myth to argue that Targum Pseudo-Jonathan reflects the original form of the Palestinian Targum and preserves the state of the targumic tradition before the second-century polemics against the angelic interpretation of Genesis 6:1–4. This theory provides a tempting solution to the puzzling reemergence of the angelic descent myth, centuries later, in early medieval texts written in the Rabbinic tradition. Several factors, however, suggest that this explanatory gloss was added at a later point in time. Most notable is this targum's close affinities with Pirqe de Rabbi Eliezer (8th–10th c.), one of our earliest extant sources to evince the reintegration of the angelic interpretation of Genesis 6:1–4 into Rabbinic exegesis ..."
Reed, Annette Yoshiko Fallen Angels and the History of Judaism and Christianity: The Reception of Enochic Literature (pp. 213-214) Cambridge University Press, 2005