1 Enoch 7:2
1 And all the others together with them took unto themselves wives, and each chose for himself one, and they began to go in unto them and to defile themselves with them, and they taught them charms and enchantments, and the cutting of roots, and made them acquainted with plants. 2 And they became pregnant, and they bare great giants, whose height was three thousand ells: 3 Who consumed all the acquisitions of men. And when men could no longer sustain them, 4 the giants turned against them and devoured mankind. 5 And they began to sin against birds, and beasts, and reptiles, and fish, and to devour one another's flesh, and drink the blood. 6 Then the earth laid accusation against the lawless ones.
Tertullian Apology 22
In fact, they call upon Satan, the demon-chief, in their execrations, as though from some instinctive soul-knowledge of him. Plato also admits the existence of angels. The dealers in magic, no less, come forward as witnesses to the existence of both kinds of spirits. We are instructed, moreover, by our sacred books how from certain angels, who fell of their own free-will, there sprang a more wicked demon-brood, condemned of God along with the authors of their race, and that chief we have referred to. It will for the present be enough, however, that some account is given of their work. Their great business is the ruin of mankind.
Notes and References
"... Whether Justin drew directly from the Book of the Watchers is impossible to say with certainty, but it seems most likely that he was dependent upon something closely akin to the work we know by that name. The book’s early composition and widespread circulation from the second century bce on (as documented in Qumran manuscripts), its early translation into Greek, its use as an authoritative source in some of the earliest Christian writings (Jude, 1 Peter, 2 Peter), and Tertullian’s express dependence on “the Scripture of Enoch” for the same traditions (On the Apparel of Women 2-3; see also Apology 22; On Idolatry 4) make it the most likely conduit of the Watchers myth to Justin. Dependence on oral transmission and written materials now lost except as embedded in the extant Enochic works cannot be excluded, but neither should it be privileged over a well-documented literary channel, especially in view of the detail and specificity with which Justin’s presentation of the myth parallels that in the Book of the Watchers ..."
Chesnutt, Randall D. "The Descent of the Watchers and its Aftermath According to Justin Martyr" in Harkins, Angela Kim, et al. (eds.) The Watchers in Jewish and Christian Traditions (pp. 167-180) Fortress Press, 2014