Calculating the pH of a salt solution of a weak base (such as pyridine hydrochloride) using its acid dissociation constant (pKa) of the conjugate acid (pyridinium ion) is a standard textbook principle of acid-base equilibria, requiring no specialized citation.
The claim states a fundamental, universally taught principle of chemical equilibrium: the pH calculation for a salt derived from a weak base and a strong acid relies on the acid dissociation constant of its conjugate acid. This is common knowledge in chemistry and does not require specific empirical papers to support it.