E0=mc^2 proof

Hello everyone
Can some one please provide (a link to) the most rigorous proof of $E_0 = mc^2$?
Actually, I got stuck on this question: why doesn't $E_0 = mc^2 + \rm{const}$? How to prove that $\rm{const} = 0$?
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 Quote by quZz Can some one please provide (a link to) the most rigorous proof of $E_0 = mc^2$?
I can provide you with the original:

http://www.fourmilab.ch/etexts/einstein/E_mc2/e_mc2.pdf
 Hi, thanks for a quick reply. In this paper it only proves that if energy of a body at rest is changed by $dE$, its mass also changes by $dm=dE/c^2$. It doesn't prove, however, a much stronger statement that all its energy equals its mass times c squared...