The splitting of transition metal d-orbitals into t2g and eg sets in an octahedral crystal field is a foundational and universally accepted principle of coordination chemistry and crystal field theory, requiring no citation.
The claim is a textbook definition of octahedral crystal field splitting (the derivation of t2g and eg energy levels from the spherical degeneracy of d-orbitals in an octahedral ligand field). It represents a fundamental chemical fact that is trivially true by definition and everyday observation in inorganic chemistry, placing it firmly in the COMMON_KNOWLEDGE category.