The Enchiridion by St. Augustine
Chapter 37.--The Same Grace is Further Clearly Manifested in This, that the Birth of Christ According to the Flesh is of the Holy Ghost.
For the same Jesus Christ who is the only-begotten, that is, the only Son of God, our Lord, was born of the Holy Ghost and of the Virgin Mary. And we know that the Holy Spirit is the gift of God, the gift being Himself indeed equal to the Giver. And therefore the Holy Spirit also is God, not inferior to the Father and the Son. The fact, therefore, that the nativity of Christ in His human nature was by the Holy Spirit, is another clear manifestation of grace. For when the Virgin asked the angel how this which he had announced should be, seeing she knew not a man, the angel answered, |The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.| And when Joseph was minded to put her away, suspecting her of adultery, as he knew she was not with child by himself, he was told by the angel, |Fear not to take unto thee Mary thy wife; for that which is conceived in her is of the Holy Ghost:| that is, what thou suspectest to be begotten of another man is of the Holy Ghost.