Catechism of the Catholic Church §2760 — excerpt
Very early on, liturgical usage concluded the Lord's Prayer with a doxology. In the Didache, we find, "For yours are the power and the glory for ever."
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: Didache 8, 2: SCh 248, 174 · Apostolic Constitutions, 7, 24, 1: PG 1,1016 · Roman Missal 22, Embolism after the Lord's Prayer