Also let us state it correctly. Einstein did not say E= m c^2. He proved it.
Yes lets state it correctly: it is E^2=p^2c^2+m^2c^4. Only when you are stationary, and so have zero momentum, does E=mc^2. Also Einstein did not prove it. He was doing physics, not maths. What he showed was that given his postulates for special relativity it followed that E^2=p^2c^2+m^2c^4. He was then proven to be correct by experiments not by the maths alone because until those experiments were done his theory might have been nothing more than an exercise in abstract maths.