The fact that free particle wavefunctions in quantum mechanics are non-normalizable plane waves requiring generalized eigenfunction expansions or rigged Hilbert spaces is a foundational textbook result in quantum mechanics and does not require specific literature citation.
The claim states a standard textbook fact of quantum mechanics: momentum eigenstates for a free particle (plane waves) are delta-function normalizable rather than square-integrable, meaning they do not belong to the standard Hilbert space L^2 but require a rigged Hilbert space. Because this is a definitional, foundational piece of standard physics curriculum well known to everyday observation and mathematical definition within the field, it is classified as common knowledge.