The use of fractional stoichiometric coefficients, such as one-half for diatomic molecules, is a standard convention in balanced chemical equations to represent reactions on a per-mole basis and requires no citation.
The claim refers to a fundamental, universally taught rule of stoichiometry in chemistry (e.g., writing H2 + 1/2 O2 -> H2O). Because this is a matter of basic definition and everyday observation in chemical education, it falls under common knowledge.