Use oslc.where=oslc:instanceShape=<URI of Specification>
NOTE the < and > are required parts of the syntax.
Fully encoded the oslc.where query parameter will look like this:
oslc.where=oslc%3AinstanceShape%3D%3Chttps%3A//jazz.ibm.com%3A9443/rm/types/OT_kFMMjpibEfC81KA6oULdRQ%3E