code: public Source retrieveSourceSynchronous(
              @NonNull @Size(min = 1) String sourceId,
              @NonNull @Size(min = 1) String clientSecret)
              throws AuthenticationException,
              InvalidRequestException,
              APIConnectionException,
              APIException {
          return retrieveSourceSynchronous(sourceId, clientSecret, null);
      }
docs: Retrieve an existing {@link Source} from the Stripe API. Note that this is a
  synchronous method, and cannot be called on the main thread. Doing so will cause your app
  to crash. This method uses the default publishable key for this {@link Stripe} instance.
  
  @param sourceId the {@link Source#mId} field of the desired Source object
  @param clientSecret the {@link Source#mClientSecret} field of the desired Source object
  @return a {@link Source} if one could be found based on the input params, or {@code null} if
  no such Source could be found.
  
  @throws AuthenticationException failure to properly authenticate yourself (check your key)
  @throws InvalidRequestException your request has invalid parameters
  @throws APIConnectionException failure to connect to Stripe's API
  @throws APIException any other type of problem (for instance, a temporary issue with
  Stripe's servers)