Matthew 1:20, 21, 22

But after he had considered this, an angel of the Lord appeared to him in a dream and said, “Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit.

She will give birth to a son, and you are to give him the name Jesus, because he will save his people from their sins.”

All this took place to fulfill what the Lord had said through the prophet