The Didache & Matthew 6:13
Most scholars date the Didache to either the first or second century AD. It’s eighth chapter therefore stands as a very early witness to the presence of a doxology at the end of the Lord’s Prayer:
1. Let not your fasts be with the hypocrites, for they fast on Mondays and Thursdays, but do you fast on Wednesdays and Fridays.
2. And do not pray as the hypocrites, but as the Lord commanded in his Gospel, pray thus: “Our Father, who art in Heaven, hallowed be thy Name, thy Kingdom come, thy will be done, as in Heaven so also upon earth; give us today our daily bread, and forgive us our debt as we forgive our debtors, and lead us not into trial, but deliver us from the Evil One, for thine is the power and the glory for ever.“
3. Pray thus three times a day.