What is the oldest written source in this country that mentions this right?
Sierra Leone
The 1991 Constitution of Sierra Leone covers freedom of association in chapter III Article 15.A.
THE CONSTITUTION OF SIERRA LEONE. International Labour Organization. https://www.ilo.org/dyn/natlex/docs/ELECTRONIC/26723/90483/F311875481/SLE26723.pdf
Constitutional History of Sierra Leone. Constitution Net. https://constitutionnet.org/country/sierra-leone