The nearest thing I see to what you seem to be saying is in Section "3. Theory of the Transformation of Coordinates and Times from a Stationary System to another System in Uniform Motion of Translation Relative to the Former”. Within that section he states:
“We now have to prove that any ray of light, measured in the moving system, is propagated with the velocity c, if, as we have assumed, this is the case in the stationary system; for we have not as yet furnished the proof that the principle of the constancy of the velocity of light is compatible with the principle of relativity.”
Is that what you are referring to?