“The equation originally expressed the equivalence of mass and energy as m =  L/c², which easily translates into the far more well-known E = mc² in Does the Inertia of a Body Depend Upon Its Energy Content? published in the Annalen der Physik (27 September 1905) : "If a body gives off the energy L in the form of radiation, its mass diminishes by L/c².”

Source: Wikiquote