That's a faulty battery. Or a background app that's going waayyy too much. Aka a possible virus. Boot your phone into safe mode. If it is still overheating while not in use for say 30ish minutes while in safe mode, it's almost certainly a bad battery. Depending on how long you've had it and the model, get the battery replaced or replace the phone. If not, you quite probably have some kind of virus.
