Catechism of the Catholic Church §1032 — excerpt
This teaching is also based on the practice of prayer for the dead, already mentioned in Sacred Scripture: "Therefore Judas Maccabeus] made atonement for the dead, that they might be delivered from their sin."
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 Lyons II (1274): DS 856 · St. John Chrysostom, Hom. in 1 Cor. 41, 5: PG 61, 361