Catechism of the Catholic Church §2744 — excerpt
Prayer is a vital necessity. Proof from the contrary is no less convincing: if we do not allow the Spirit to lead us, we fall back into the slavery of 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: St. John Chrysostom, De Anna 4, 5: PG 54, 666 · St. Alphonsus Liguori, Del gran Mezzo della preghiera