Catechism of the Catholic Church §2151 — excerpt
Rejection of false oaths is a duty toward God. As Creator and Lord, God is the norm of all truth. Human speech is either in accord with or in opposition to God who is Truth itself.
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.