WordPress get auto updated on Cpanel servers running PHP in suPHP mode. If PHP is running as mod_php (DSO), then you have to enter FTP details in wp-config.php

Add following to wp-config.php

define('FS_METHOD', 'ftpext');
define('FTP_BASE', '/home/USERNAME/public_html/');
define('FTP_USER', 'USERNAME');
define('FTP_PASS', 'PASSWORD_HERE');
define('FTP_HOST', 'localhost');
define('FTP_SSL', false);

Now update will work with out prompting for FTP password.