What is the oldest written source in this country that mentions this right?
Estonia
The first assertion of a right to association in Estonia is in the Estonian Declaration of Independence, written on February 23, 1918. The right is asserted as the third of the numbered principles in the Declaration. (“Estonian Declaration of Independence”, 1918)
“Estonian Declaration of Independence.” 1918. Declaration Project. February 23. https://declarationproject.org/?p=1725.