diff --git a/spell.h b/spell.h
index daf6ea1..2c278fe 100644
--- a/spell.h
+++ b/spell.h
@@ -1,3 +1,7 @@
+/*
+  This code belongs to https://rosettacode.org/wiki/Number_names
+*/
+
 #include <string>
 #include <iostream>
 using std::string;