What is the oldest written source in this country that mentions this right?
Antigua and Barbuda
Freedom of association is asserted in Chapter II, article 13(1) of the Antigua and Barbuda Constitution of 1981.
References:
The 1981 Constitution of Antigua and Barbuda: https://www.legislation.gov.uk/uksi/1981/1106/pdfs/uksi_19811106_en.pdf