The claim that an atomic nucleus can exist with more protons than neutrons is a matter of basic scientific fact and everyday observation in nuclear physics (for instance, the Helium-3 nucleus has two protons and one neutron, and Hydrogen-1 has one proton and zero neutrons). No citation is needed.
The claim is a well-established physical fact about atomic nuclei (e.g., Helium-3, Hydrogen-1, and various proton-rich unstable isotopes near the proton drip line). It represents common foundational knowledge in nuclear physics rather than a disputed hypothesis.