The doxology closing the prayer in memory and public recitation, "For thine is the kingdom, and the power, and the glory, for ever. Amen" (Matt 6:13, KJV), is absent from the oldest Greek manuscripts (Sinaiticus, Vaticanus) and from Luke entirely; ASV and most modern translations print it only as a margin note. The textual judgment is stated plainly by one of the standard 19th-century evangelical commentaries:
"If any reliance is to be placed on external evidence, this doxology, we think, can hardly be considered part of the original text." — Jamieson, Fausset & Brown, Commentary Critical and Explanatory on the Whole Bible (1871), on Matthew 6:13
That the church added it, not that Jesus spoke it, is the near-consensus text-critical judgment (cite-only), and it is not a loss. Its wording is David's own doxology: "Thine, O Jehovah, is the greatness, and the power, and the glory, and the victory, and the majesty... thine is the kingdom, O Jehovah" (1 Chr 29:11, ASV). The earliest worshiping church, closing the Lord's prayer, reached for the language Israel already used to close its own, fitting even if not Christ's own words, and labeled honestly here rather than silently harmonized into the text.