diff --git a/postgres/sleeping.md b/postgres/sleeping.md
index 4ca08f8..9fe5482 100644
--- a/postgres/sleeping.md
+++ b/postgres/sleeping.md
@@ -30,3 +30,5 @@ Time: 0.260 ms
 ```
 
 As you'll notice, the `pg_sleep` statement took about 5 seconds.
+
+[source](http://www.if-not-true-then-false.com/2010/postgresql-sleep-function-pg_sleep-postgres-delay-execution/)