Catechism of the Catholic Church §1225 — excerpt
In his Passover Christ opened to all men the fountain of Baptism. He had already spoken of his Passion, which he was about to suffer in Jerusalem, as a "Baptism" with which he had to be baptized.
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: St. Ambrose, De sacr. 2, 2, 6: PL 16, 444