What is the oldest written source in this country that mentions this right?
Togo
The first assertion of the freedom of religion in Togo would be the Constitution de la République Togolaise, 5 mai 1963 (5th of May, 1963).
https://www.loc.gov/resource/amedscd.2008700247/?sp=7&r=-1.395,-0.017,3.789,1.703,0