Catechism of the Catholic Church §1554 — excerpt
"The divinely instituted ecclesiastical ministry is exercised in different degrees by those who even from ancient times have been called bishops, priests, and deacons." Catholic doctrine, expressed in the liturgy, the Magisterium, and the constant practice of the Church, recognizes that there are two degrees…
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: LG 28 · St. Ignatius of Antioch, Ad Trall. 3,1: SCh 10, 96