This is a fundamental definition in quantum mechanics (maximally entangled pure states have purity 1, while maximally mixed states have a purity of 1/d), so no citation is needed.
The claim states a fundamental definition in quantum information theory: pure maximally entangled states have a density matrix purity of 1 (or von Neumann entropy of 0 for subsystems in certain contexts), whereas maximally mixed states have the lowest possible purity (1/d). Because this is a trivial mathematical definition inherent to the formalisms of quantum mechanics, it falls under common knowledge.