ABSPATH not working! Any idea why?

Question

This is what in my wp-config.php :

if ( !defined('ABSPATH') )
    define('ABSPATH', dirname(__FILE__) . '/');

I am calling from plugin/pluginName directory to :

require_once( ABSPATH . 'wp-includes/user.php');

But it is returning:

Warning: require_once(ABSPATHwp-includes/user.php) [function.require-once]: failed to open stream: No such file or directory in /home/------/wp-content/plugins/---/---.php on line 43

Fatal error: require_once() [function.require]: Failed opening required 'ABSPATHwp-includes/user.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/------/wp-content/plugins/---/---.php on line 43

Any idea why I am having this error? Any solution? Thanks.