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.

Date: 200-50 B.C.E.
* Dates are based on scholarly estimates

Josephus Antiquities of the Jews 1.3

1 Now this posterity of Seth continued to esteem God as the Lord of the universe, and to have an entire regard to virtue, for seven generations: but in process of time they were perverted, and forsook the practices of their fore-fathers; and did neither pay those honours to God which were appointed them, nor had they any concern to do justice towards men. But for what degree of zeal they had formerly shewn for virtue, they now shewed by their actions a double degree of wickedness. Whereby they made God to be their enemy. For many Angels of God accompanied with women, and begat sons that proved unjust, and despisers of all that was good; on account of the confidence they had in their own strength. For the tradition is, that these men did what resembled the acts of those whom the Grecians call Giants. But Noah was very uneasy at what they did: and being displeased at their conduct, persuaded them to change their dispositions, and their actions for the better. But seeing they did not yield to him, but were slaves to their wicked pleasures, he was afraid they would kill him, together with his wife and children, and those they had married. So he departed out of that land.

Date: 93-94 C.E.
* Dates are based on scholarly estimates