What is the oldest written source in this country that mentions this right?
Kuwait
The first assertion of the right to freedom of religion in Kuwait is in the Constitution of 1962, ratified on November 11, 1962. The right can be found in Part III, Article 35. Part 1, Article 2 establishes the official religion as Islam (“Kuwait 1962 Constitution”, 1962).
“Kuwait 1962 Constitution.” 1962. Constitute Project. November 11. https://www.constituteproject.org/constitution/Kuwait_1992?lang=en.