Catechism of the Catholic Church §2178 — excerpt
This practice of the Christian assembly dates from the beginnings of the apostolic age. The Letter to the Hebrews reminds the faithful "not to neglect to meet together, as is the habit of some, but to encourage one another."
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: Sermo de die dominica 2 · 6: PG 86/1, 416C · 421C