Eh, this is a very tricky answer. There is a force, the strong atomic force, that holds nuclei together against the repelling force of the like charged protons in the core. And its force field is made up of gluons...mediator bosons that carry the SAF among the quarks within the nuclei.
Now here's what's tricky. When atoms split or fuse, the resulting products end up with slightly less mass than the total mass the fission or fusion started with. That slightly less mass is called the mass deficiency, dm. And it's from that deficiency that the fission and fusion energy comes; as in de = dm c^2.
So where'd that lost mass, dm, come from? Do the nucleons lose mass? No. The protons and neutrons don't change mass. And the electrons whirling about the nuclei don't change mass either.
So where'd that lost mass, in de = dm c^2 come from?
It comes from the strong atomic force and the energy it exhibits. And what we have is an example of dm = de/c^2 where the energy of the gluons represents a mass equivalent, like Einstein predicted. And that's the mass deficiency. The lost mass.energy equivalent of the forces where? In between the mass particles, in the space between the quarks.
So they were both right. Tesla for identifying the mass defect from in between the quarks as the energy source and Einstein for recognizing that the strong atomic force has a mass equivalent, dm = de/c^2.