Catechism of the Catholic Church §956 — excerpt
The intercession of the saints. "Being more closely united to Christ, those who dwell in heaven fix the whole Church more firmly in holiness.... They do not cease to intercede with the Father for us, as they proffer the merits which they acquired on earth through the one mediator between God and men, Christ Jesus....
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 49 · St. Dominic, dying, to his brothers · St. Therese of Lisieux, the Final Conversations, tr. John Clarke (Washington: ICS, 1977), 102