Catechism of the Catholic Church §200 — excerpt
These are the words with which the Niceno-Constantinopolitan Creed begins. the confession of God's oneness, which has its roots in the divine revelation of the Old Covenant, is inseparable from the profession of God's existence and is equally fundamental.
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: Roman Catechism I, 2, 2