Catechism of the Catholic Church §1118 — excerpt
The sacraments are "of the Church" in the double sense that they are "by her" and "for her." They are "by the Church," for she is the sacrament of Christ's action at work in her through the mission of the Holy Spirit.
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. Augustine, De civ. Dei, 22, 17: PL 41, 779 · St. Thomas Aquinas, STh III, 64,2 ad 3