This is a definitional statement about Karl Popper's theory of critical rationalism and does not require empirical citation.
The claim outlines the foundational tenet of Karl Popper's critical rationalism (falsifiability versus verifiability). This is a well-established philosophical definition and textbook fact about the philosophy of science rather than a contested empirical hypothesis requiring specific papers to verify. Therefore, it is categorized as common knowledge for the purposes of philosophy of science definitions.