Where does the "For the Kingdom" come from?

The earliest source of this "doxology" comes from the first century and the Didache. In this document about the teachings and practices of the apostles and early church, this phrase comes immediately after the Lord's Prayer ("Our Father"). The source for the doxology comes from sacred scripture, inthe New Revised Standard Version they would be:

1 Chronicles 29:11
Yours, O Lord, are the greatness, the power, the glory, the victory, and
the majesty; for all that is in the heavens and on the earth is yours;
yours is the kingdom, O Lord, and you are exalted as head above all.

Daniel 2:37
You, O king, the king of kings--to whom the God of heaven has given the
kingdom, the power, the might, and the glory,

The periti at the Second Vatican Council wanted to reattach the "For the Kingdom" to the "Lord's Prayer." However, Pope Paul VI added the embolism between the two prayers.

In The Mystery of Faith, by Lawrence J. Johnson (published by FDLC 2004, in Washington), the following is stated on page 99:

The Byzantine rite traditionally concludes the Lord's Prayer with the acclamation 'For the kingdom, the power, and the glory'....This doxology seemingly resulted from the desire to end the prayer with a more positive statement than 'deliver us from evil.' The antiquity of this acclamation is evident since it is found in some biblical manuscripts, probably as a result of liturgical usage.