Catechism of the Catholic Church §2704 — excerpt
Because it is external and so thoroughly human, vocal prayer is the form of prayer most readily accessible to groups. Even interior prayer, however, cannot neglect vocal prayer.
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. Teresa of Jesus, the Way of Perfection 26, 9 in the Collected Works of St. Teresa of Avila, tr. K. Kavanaugh, OCD, · O. Rodriguez, OCD (Washington DC: Institute of Carmelite Studies, 1980), II, 136