What is the oldest written source in this country that mentions this right?
Guinea
The first assertion of the right to associate in Guinea was in their 1958 Constitution, ratified on November 10th. The right is asserted in Title X, Section 40 (Maury, “République de Guinée, Constitution du 10 Novembre 1958”, 1958).
Maury, Jean-Pierre. 1958. “République de Guinée, Constitution Du 10 Novembre 1958.” République de Guinée, Constitution Du 10 Novembre 1958, Digithèque MJP. November 10. https://mjp.univ-perp.fr/constit/gn1958.htm.