The relationship between Mass and (Kinetic) Energy in General(?) Relativity

https://en.wikipedia.org/wiki/Mass

"In Newtonian physics, mass can be generalized as the amount of matter in an object. However, at very high speeds, special relativity states that the kinetic energy of its motion becomes a significant additional source of mass"

At what stage in the theory of (General? Special?) Relativity does it become apparent that the above statement applies?

I am assuming that before Einstein this relationship was not known. Did it just fall out of the equations or was it some sort of an axiom that was shown to be correct?