What is the oldest written source in this country that mentions this right?
Lesotho
Freedom of expression was asserted in the 1966 Lesotho Constitution, as well as in the 2010 Lesotho Constitution.
References:
1966 Lesotho Constitution: O’LEARY, B. L. “THE CONSTITUTION OF LESOTHO: AN OUTLINE.” The Comparative and International Law Journal of Southern Africa 1, no. 2 (1968): 266–70. http://www.jstor.org/stable/23240737.
1993 Lesotho Constitution:https://www.constituteproject.org/constitution/Lesotho_2018.