Catechism of the Catholic Church §877 — excerpt
Likewise, it belongs to the sacramental nature of ecclesial ministry that it have a collegial character. In fact, from the beginning of his ministry, the Lord Jesus instituted the Twelve as "the seeds of the new Israel and the beginning of the sacred hierarchy."
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: AG 5