In this case there is no "before the Bible." This is a rough timeline of New Testament texts, nowadays generally agreed on by most scholars with the occasional disagreement or tweak to the details (all dates are CE):

[Crucifixion: ~30-35]

45-60: Genuine letters of Paul

65-75: Gospel of Mark

65-90: Ephesians, Colossians, and 2 Thessalonians (or these could be in the 2nd century)

80-85: Gospel of Matthew

90-115: Gospel of Luke, Gospel of John, Johannine letters, Revelation (sometimes Rev is put later)

95-120: Acts

early 2nd century: everything else

There are no earlier Christian texts. The next earliest set are what are known as the Apostolic Fathers, which include the letters of Clement of Rome, the letters of Ignatius, and a few other things, none of which can be reliably dated before the 90s. Some people date at least portions of the Gospel of Thomas in this period, and there's a lot of speculation about earlier versions of the Gospels, but, as written, not a single Christian text can reliably be dated before the 40s, and, except for a few outliers who date Mark very early, most people think that the earliest surviving Christian text is 1 Thessalonians. There are also no definitively Christian archaeological findings from this period, so text is the only evidence we have.

What that means, to answer your question, is that the very earliest mention of the virgin birth of Jesus is the Gospel of Matthew. People used to assume that the author of Mark accepted the tradition and just didn't include it, but the scholarship is quietly moving away from that. In my opinion there's better evidence that if he knew the tradition he didn't accept it. Mark begins in Jesus's adulthood, and in chapter 1 John the Baptist, who knows that Jesus is the Messiah, doesn't give any indication that he had a divine birth. Chapter 6 of Mark introduces us to Jesus's family, and the narrator doesn't do anything to correct the assumption that Jesus's father is a carpenter. At the very least, I think that the burden of proof lies with those who would like Mark to have known the tradition. Paul says that Jesus was "born of a woman" (Gal 4:4), but the Greek word is just generic "woman" (gyne). Also, none of the best-accepted reconstructions of earlier versions or sources of the Gospels include the virgin birth.

There's a lot to say about Greek and Roman miraculous birth traditions in general, and I know scholars who argue that there was no such tradition for Jesus until Matthew made use of the genre--that is, Matthew innovated the story using traditions about Heracles and other heroes, rather than included a live tradition about Jesus--but there's no definitive evidence either way.

For the record, the Gospel of James, which gives a detailed narrative of the early life of Mary, including both the immaculate conception (which is the conception of Mary by her parents, not the conception of Jesus) and the virgin birth, is usually dated to after 150.