Catechism of the Catholic Church §2153 — excerpt
' But I say to you, Do not swear at all.... Let what you say be simply 'Yes' or 'No'; anything more than this comes from the evil one."
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.