The first mention in the Old Testament that the coming Christ would be born of a virgin occurs right at the beginning.
And I will put enmity between thee and the woman, and between thy seed and her seed: it shall bruise thy head, and thou shalt bruise his heel.
Genesis 3:15
This prophecy, known as the Protevangelium, comes from the most ancient oracle known to man, the oracle that the Lord pronounced when He found our first parents, Adam and Eve, guilty of sin. The Lord is speaking to Satan, who has enticed “the woman,” Eve, into disobeying the Lord’s command against eating fruit from the Tree of the Knowledge of Good and Evil. He is saying that Satan will someday be crushed and utterly defeated by the seed of the woman.
The pronoun used to designate the seed is “his” (in “his heel”). In place of “it” (in “it shall bruise”), the more accurate translation is “he” (1). Therefore, the coming conqueror must be a single man. But why is He called the seed of a woman? A child is ordinarily regarded as the seed of his father and forefathers. The striking and unnatural character of the expression “her seed” suggests that it is a uniquely fitting name for the victor over Satan. Unlike other men, He would be the seed of a woman only. He would not be a man’s seed. A virgin would conceive Him without losing her virginity.
Isaiah’s Oracle
Over seven hundred years before Jesus was born, the prophet Isaiah enlarged upon the Protevangelium.
Therefore the Lord himself shall give you a sign: Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel.
Isaiah 7:14
The name Immanuel means, “God with us.” Isaiah is appending to Genesis 3:15 the information that the virgin-born conqueror of Satan would be God Himself in the flesh.
Matthew 1.20
Matthew 1:21–23 She will bear a son, and you shall call his name Jesus, for he will save his people from their sins.” 22 All this took place to fulfill what the Lord had spoken by the prophet:
23 “Behold, the virgin shall conceive and bear a son, and they shall call his name Immanuel”
https://www.desiringgod.org/messages/the-fall-of-satan-and-the-victory-of-christ