I suppose you have some WordPress installed and someone is trying to access to wp-config-sample.php
If you check your logs, you will notice that username_here attempts to connect to your database at the same time someone tries to access yourdomain/wp-config-sample.php
grep -r wp-config-sample.php /var/log/
If you want to avoid having those failed records in your logs, you can remove the wp-config-sample.php file from your WordPress installation. If WordPress is already installed and configured, this file is no longer needed.