The superposition principle is one of the founding principles of quantum theory. Spatial quantum superpositions have so far been tested only with small systems, from photons and elementary particles to atoms and molecules. Such superpositions for massive objects have been a long-standing sought-after goal. This is important not only in order to confirm quantum theory in new regimes, but also in order to probe the quantum-gravity interface. In addition, such an experiment will enable to test exotic theories, and may even enable new technology. Creating such superpositions is notoriously hard because of environmental decoherence, whereby the large object couples strongly to the environment which turns the delicate quantum state into a statistical mixture (classical state). However, advances in the technology of isolation could in future suppress such decoherence. Here we present a decoherence channel which is not external but internal to the object, and consequently improved isolation would not help. This channel originates from the phonons (sound waves) within the object. We show that such phonons are excited as part of any splitting process, and thus we establish a fundamental and universal limit on the possibility of future spatial quantum superpositions with massive objects.