In the abstract of a scientific report, when comparing an experimentally measured value to a current accepted value, should you provide a citation for it?
It is typically not necessary to report the currently accepted value of a quantity in the abstract, but, indeed, you should report it in the scientific report (with citation). In the report you should also discuss the compatibility between your result and the currently accepted value.
However, if there is a significant discrepancy between the accepted value and your result, and the main aim of your report is to discuss the cause of this discrepancy, then it can be useful to report the accepted value also in the abstract. In this case, the citation is needed, but take into account that, frequently, citations in abstracts follow different style conventions from those of the main text.
Pay particular attention to the source of the accepted value. For example, for fundamental physical constants, the Committee on Data for Science and Technology (CODATA) publishes around every 4 years a new "adjustment" of the recommended values (here you can find the values of the latest adjustment with references). These recommended values are calculated on the basis of the best available measurements. The CODATA adjustment should be considered the only reliable source for the accepted values of fundamental physical constants (values reported in books are typically outdated). Other official sources exist for values of quantities related to other fields.