How to cite computer terminals like Bloomberg or Thomson Reuters?
Unless there is a unique, persistent link to the statement from the rating agency - e.g. a link to their archive of ratings, etc, you cannot use that information as a citation. You should imagine the reader far in the future, reading and trying to verify your work - what would they consult, if anything in order to validate your statements, or reproduce your work. 

If the agencies do not maintain a public archive, you may be able to take a screenshot of the terminal, and publish it to a place which will keep a persistent public archive of it, e.g. figshare.