What is the oldest written source in this country that mentions this right?

Latvia

Article 96 of the 1922 constitution states “Everyone has the right to inviolability of his or her private life, home and correspondence” (Constitute Project, “Latvia 1922, reinst. 1991, rev. 2016”).

https://constituteproject.org/constitution/Latvia_2016?lang=en

Privacy Rights · Country sources (all rights) · Years mentioned: 1922, 1991, 2016 · Last edited 2022-12-28 · Edit on GitHub