The pH of a salt derived from a weak acid and a weak base depends on the relative dissociation constants (Ka and Kb) of the parent acid and base, a foundational principle of aqueous acid-base chemistry that requires no citation.
The claim describes a fundamental chemical truism taught universally in introductory chemistry regarding salt hydrolysis, where the resulting pH is determined by the relative strength (dissociation constants) of the weak conjugate acid and weak base ions. As a matter of definition and direct derivation from chemical equilibrium laws, this is classified as common knowledge.