What is the oldest written source in this country that mentions this right?
Lebanon
The first assertion of freedom of religion in Lebanon was in their 1926 Constitution, ratified on May 23. Articles 9, 10, 19, and 22 grant religious freedom, equality, and prohibit religious discrimination. (“The Lebanese Constitution”, 1926).
1926. The Lebanese Constitution. Lebanese Parliament. May 23. https://www.lp.gov.lb/backoffice/uploads/files/Lebanese%20%20Constitution-%20En.pdf.