References in Executive Summary?
My general answer would be: No. Summaries should summarize the work/report and anyone reading the summary will not need to go to references to check on sources since those are given in the report itself. I can see exceptions from this if one or two references are key to the report by for example, being ground-breaking or by being in conflict. Then it may be important to show from where the key aspects originate and to perhaps list the single r couple of references after the text. But, the persons reading the summary may not be familiar with referencing so make sure you know your intended audience and the style they are familiar with. It may still not be necessary to provide references but to reference the material by stating the author name(s) and the place of publication without reference details. Again, the report summarized in your Exec. Summary will have the details.