Catechism of the Catholic Church §1471 — excerpt
The doctrine and practice of indulgences in the Church are closely linked to the effects of the sacrament of Penance.
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: Paul VI, apostolic constitution, Indulgentiarum doctrina, Norm 1 · Indulgentiarum doctrina, Norm 2 · Norm 3