diff --git a/singleton/src/main/java/com/iluwatar/EnumIvoryTower.java b/singleton/src/main/java/com/iluwatar/EnumIvoryTower.java
index e9c3dc4dc..ad78cb855 100644
--- a/singleton/src/main/java/com/iluwatar/EnumIvoryTower.java
+++ b/singleton/src/main/java/com/iluwatar/EnumIvoryTower.java
@@ -3,6 +3,7 @@ package com.iluwatar;
 /**
  *
  * Enum Singleton class.
+ * http://stackoverflow.com/questions/70689/what-is-an-efficient-way-to-implement-a-singleton-pattern-in-java
  *
  */
 public enum EnumIvoryTower {