Catechism of the Catholic Church §253 — excerpt
The Trinity is One. We do not confess three Gods, but one God in three persons, the "consubstantial Trinity". e. by nature one God." , the divine substance, essence or nature."
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 Constantinople II (553): DS 421 · Council of Toledo XI (675): DS 530:26 · Lateran Council IV (1215): DS 804