What is the oldest written source in this country that mentions this right?
Fiji
Freedom of expression was first protected by Article II, Section 3 of the 1970 constitution of Fiji.
References:
1970 Constitution of Fiji: Peaslee Amos J.; Xydis, Dorothy Peaslee. Constitutions of Nation. The Hague, Martinus Nijhoff. https://heinonline-org.proxygw.wrlc.org/HOL/Page?collection=cow&handle=hein.cow/ctituson0002&id=136&men_tab=srchresults