What is the oldest written source in this country that mentions this right?
Zimbabwe
Zimbabwe first highlights the right of association in section 21. Protection of freedom of assembly and association of the 1980 constitution after independence from the United Kingdom.
Zimbabwe. 1980. “CONSTITUTION of ZIMBABWE.” AceProject. 1980. https://aceproject.org/ero-en/regions/africa/ZW/Constitution%20of%20Zimbabwe%201980.pdf.