Catechism of the Catholic Church §1393 — excerpt
Holy Communion separates us from sin. the body of Christ we receive in Holy Communion is "given up for us," and the blood we drink "shed for the many for the forgiveness of sins."
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. 4, 6, 28: PL 16, 446