Catechism of the Catholic Church §496 — excerpt
From the first formulations of her faith, the Church has confessed that Jesus was conceived solely by the power of the Holy Spirit in the womb of the Virgin Mary, affirming also the corporeal aspect of this event: Jesus was conceived "by the Holy Spirit without human seed".
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: Council of the Lateran (649): DS 503 · DS 10-64 · St. Ignatius of Antioch, Ad Smyrn 1-2: Apostolic Fathers, ed. J. B. Lightfoot (London: Macmillan, 1889), 11/2, 289-293 · SCh 10, 154-156