What is the oldest written source in this country that mentions this right?
Dominican Republic
Article 30 of the 1844 Constitution of the Dominican Republic guarantees freedom of association.
References:
1844 Constitution of the Dominican Republic: Spanish orignal text of the Constitution of 1844 57 (2012) Chapter I: Of Sovereignty https://heinonline-org.proxygw.wrlc.org/HOL/Page?collection=cow&handle=hein.cow/zzdo0015&id=5&men_tab=srchresults