When was the Doxology added to the Lord’s Prayer?
Jesus taught the Lord’s Prayer twice during his earthly ministry (cf Mt 6:9ff. and Luke 11:2ff.). When he taught the Lord’s Prayer to his disciples (the Luke account), he did not include the doxology. When he taught the Lord’s Prayer as part of the Sermon on the Mount (the Mt account), there is some question whether he included the doxology or not.
In the Matthew account, the earliest Greek manuscript to include the doxology is dated in the 400’s. There are a few Greek manuscripts that are a little earlier than this (the 300’s) that do not have the doxology. However, there are some versions (Bible translations) such as a Latin translation, a Coptic translation, and a Syriac translation (which are as early as the earliest Greek manuscripts) that do include the doxology.