The statement that neutralized solutions of strong acids and strong bases have a pH of exactly seven at standard conditions is a matter of definition and fundamental acid-base stoichiometry, requiring no citation.
The claim is a basic chemical truism based on the self-ionization constant of water (Kw = 1.0 x 10^-14 at standard conditions) where the equivalence point of a strong acid-strong base titration yields equal concentrations of hydronium and hydroxide ions, resulting in a pH of 7. Therefore, it is evaluated as common knowledge and does not require literature citation.