What is the oldest written source in this country that mentions this right?
Eswatini
The Constitution of Eswatini, ratified October 4, 2004, was the first official document to protect association. The right is protected under Chapter III, Article 25 (“Eswatini 2005 Constitution”, 2004).
“Eswatini 2005 Constitution.” 2004. Constitute. October 4. https://www.constituteproject.org/constitution/Swaziland_2005?lang=en.