What is the oldest written source in this country that mentions this right?
Uruguay
The Constitution of the Eastern Republic of Uruguay was ratified in 1830. Article 5 grants freedom of religion. Article 5 declares the state supports no religion.
Constitution Project. “Uruguay 1966 (Reinst. 1985, Rev. 2004) Constitution.” Constitute. POGO, April 27, 2022. Last modified April 27, 2022. Accessed July 5, 2022. https://www.constituteproject.org/constitution/Uruguay_2004?lang=en.