The statement that bosonic Fock space is isomorphic to the state space of a collection of quantum harmonic oscillators is a standard definition and fundamental principle of quantum mechanics and quantum field theory; no citation is needed.
The claim states a foundational isomorphism in quantum mechanics and quantum field theory: the Fock space for identical bosons is mathematically equivalent (isomorphic) to the direct sum of tensor products of Hilbert spaces of quantum harmonic oscillators (representing the occupation number states of each mode). This is a textbook definition and mathematical identity rather than an empirical hypothesis requiring new research citations, making it common knowledge in the context of mathematical and quantum physics.