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

Finland

The Finnish right to association was first enshrined on July 17th, 1919 in the Constitution Act of Finland. The right is found in Section 10a. (“Constitution Act of Finland”, 1919).

“Constitution Act of Finland.” 1919. Refworld. July 17. https://www.refworld.org/docid/3ae6b53418.html.

Freedom of Association · Country sources (all rights) · Last edited 2023-12-11 · Edit on GitHub