What is the oldest written source in this country that mentions this right?
Greece
The Greek Constitution of 1927 established the right to association, being ratified on June 3. The right to association is found in Chapter III, Article 14 (“Constitution of Greece (1927)”, 2022).
“Constitution of Greece (1927).” 2022. Wikisource, the Free Online Library. Wikimedia Foundation, Inc. May 16. https://en.wikisource.org/wiki/Constitution_of_Greece_(1927)