diff --git a/source/api/filter.md b/source/api/filter.md
index acd44c6a1c..64f5be22b6 100644
--- a/source/api/filter.md
+++ b/source/api/filter.md
@@ -212,15 +212,8 @@ hexo.extend.filter.register("after_clean", function () {
 
 Add middleware to the server. `app` is a [Connect] instance.
 
-For example, to add `X-Powered-By: Hexo` to the response header:
-
-```js
-hexo.extend.filter.register("server_middleware", function (app) {
-  app.use(function (req, res, next) {
-    res.setHeader("X-Powered-By", "Hexo");
-    next();
-  });
-});
-```
+This hook is provided by [hexo-server], not Hexo core, so it only runs when `hexo-server` is installed. See the [hexo-server README] for an implementation example.
 
 [Connect]: https://github.com/senchalabs/connect
+[hexo-server]: https://github.com/hexojs/hexo-server
+[hexo-server README]: https://github.com/hexojs/hexo-server#middleware