The use of reference compounds (such as TMS) to define NMR chemical shifts is a fundamental and standard practice in spectroscopy, requiring no citation.
The claim that NMR chemical shifts can be defined relative to different reference compounds is a standard, fundamental fact of analytical chemistry and spectroscopy (e.g., referencing to TMS, DSS, or solvent signals depending on the nucleus and conditions). According to the instructions, a trivially true statement of direct observation or established definition is classified as COMMON_KNOWLEDGE, requiring no citations.