code: public Source retrieveSourceSynchronous(
              @NonNull @Size(min = 1) String sourceId,
              @NonNull @Size(min = 1) String clientSecret,
              @Nullable String publishableKey)
              throws AuthenticationException,
              InvalidRequestException,
              APIConnectionException,
              APIException {
          String apiKey = publishableKey == null ? mDefaultPublishableKey : publishableKey;
          if (apiKey == null) {
              return null;
          }
          return mApiHandler.retrieveSource(sourceId, clientSecret, apiKey, mStripeAccount);
      }
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.
  
  @param sourceId the {@link Source#mId} field of the desired Source object
  @param clientSecret the {@link Source#mClientSecret} field of the desired Source object
  @param publishableKey a publishable API key to use
  @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)