The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes, distinguishing two kinds of transfer of energy, as heat and as thermodynamic work, and relating them to a function of a body’s state, called internal energy.
The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed from one form to another, but can be neither created nor destroyed.
Mass and energy are equivalent and there are a gazillion experiments showing that.
It is well known that a U235 nucleus has a greater mass than do the constituent products when it decays through spontaneous fission. The difference in masses exactly equals the kinetic energy of the decay fragments and the energy of any gamma rays emitted.