Central banks backing fiat currency as legal tender for settling debts is a definitional and institutional feature of modern monetary systems, making citations largely unnecessary for the basic premise, though legal frameworks extensively discuss its implementation.
The claim is a fundamental characteristic of fiat currency systems managed by central banks. While no paper is solely dedicated to proving this foundational definition as its primary research question, several papers naturally describe fiat and central bank digital currencies as legal tender used for transactions and settlements. Consequently, the claim is common knowledge, supported by background legal and economic descriptions in the literature.