1 Enoch 6:2
1 And it came to pass when the population of humans had increased during those times, beautiful and attractive daughters were born to them. 2 And the angels, the children of heaven, saw them and desired them, and said to each other: 'Come, let us choose wives from among the humans and father children.' 3 And Semjâzâ, their leader, said to them: 'I fear that you will not actually agree to do this, and I alone will have to pay the penalty of a great sin.' 4 And they all replied to him and said: 'Let us all take an oath, and all bind ourselves with a solemn promise not to abandon this plan but to carry out this act.' 5 Then they all took an oath together and bound themselves with a solemn promise to do so.
Eusebius Preparation for the Gospel 5.4
And this argument is still further confirmed by Plutarch, in the passage where he says that the mythical narratives told as concerning gods are certain tales about daemons, and the deeds of Giants and Titans celebrated in song among the Greeks are also stories about daemons, intended to suggest a new phase of thought. Of this kind then perhaps were the statements in the Sacred Scripture concerning the giants before the Mood, and those concerning their progenitors, of whom it is said, 'And when the angels of God saw the daughters of men that they were fair, they took unto them wives of all that they chose,' and of these were born 'the giants the men of renown which were of old.' For one might say that these daemons are those giants, and that their spirits have been deified by the subsequent generations of men, and that their battles, and their quarrels among themselves, and their wars are the subjects of these legends that are told as of gods.
Notes and References
"... Origen initially accepted 1 Enoch as scripture, but later rejected the Enochic writings. The rejection implies that 1 Enoch was regularly read in some churches. Earlier, Origen calls the Enochic writings “scripture” (On First Principles 1.3; 4.35). After the death of Origen, orthodox writers seldom refer to 1 Enoch for more than two centuries, Lactantius and Eusebius being the primary exceptions. It is not precisely clear when the Enochic writings were specifically excluded by name. Generally speaking, however, after Origen there are few positive citations of the Enochic literature in subsequent Church literature, although the volume continued in the Ethiopian biblical canon ..."
McDonald, Lee Martin "The Parables of Enoch in Early Christianity" in Charlesworth, James H., and Darrell L. Bock., ed. Parables of Enoch: A Paradigm Shift (p. 369) T&T Clark, 2013