Catechism of the Catholic Church §957 — excerpt
Communion with the saints. "It is not merely by the title of example that we cherish the memory of those in heaven; we seek, rather, that by this devotion to the exercise of fraternal charity the union of the whole Church in the Spirit may be strengthened.
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 50 · Martyrium Polycarpi, 17: Apostolic Fathers II/3, 396