111 years ago, Albert Einstein send on ’Annalen der Physik’ the 4th of his Annus Mirabilis papers, a three-page paper titled: “Does the Inertia of a Body Depend Upon Its Energy Content?” [PDF, wikipedia], from which it’s can be derived the famous relation E = mc².
Actually you can not find such equation in the article and rather you will find this other one, a bit ugliest (and approximate, magnitudes of fourth and higher orders are neglected):
E = mc² is not the same equation but still serves at least to illustrate the equivalence relationship between mass and energy (through the gargantuan factor c²), and after all Einstein indeed explicitly wrote down this relation in the last page, but using words:
If a body gives off the energy L in the form of radiation, its mass diminishes by L/c².
in any case, is good to know the complete history/formula, and Sixty Symbols has a short video about this thing: