diff --git a/docs/04-certificate-authority.md b/docs/04-certificate-authority.md
index b9cb74e..253e5fe 100644
--- a/docs/04-certificate-authority.md
+++ b/docs/04-certificate-authority.md
@@ -26,7 +26,6 @@ openssl req -new -key ca.key -subj "/CN=KUBERNETES-CA" -out ca.csr
 # Self sign the csr using its own private key
 openssl x509 -req -in ca.csr -signkey ca.key -CAcreateserial  -out ca.crt -days 1000
 ```
-
 Results:
 
 ```
@@ -34,6 +33,8 @@ ca.crt
 ca.key
 ```
 
+Reference : https://kubernetes.io/docs/concepts/cluster-administration/certificates/#openssl
+
 The ca.crt is the Kubernetes Certificate Authority certificate and ca.key is the Kubernetes Certificate Authority private key.
 You will use the ca.crt file in many places, so it will be copied to many places.
 The ca.key is used by the CA for signing certificates. And it should be securely stored. In this case our master node(s) is our CA server as well, so we will store it on master node(s). There is not need to copy this file to elsewhere.