Catechism of the Catholic Church §469 — excerpt
The Church thus confesses that Jesus is inseparably true God and true man. He is truly the Son of God who, without ceasing to be God and Lord, became a man and our brother: "What he was, he remained and what he was not, he assumed", sings the Roman Liturgy.
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: LH, 1 January, Antiphon for Morning Prayer · St. Leo the Great, Sermo in nat. Dom. 1, 2 · PL 54, 191-192 · Liturgy of St. John Chrysostom, Troparion O monogenes