MYSQL(DYNAMIC QUERY)

Question

ET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(' sum (case when DATE_FORMAT(message_accepted_by_signalmash, '%d')= ''',da,
      ''' then msg_count else 0 end ) AS  `',da,'`')
   )INTO @sql
FROM(SELECT  DATE_FORMAT(message_accepted_by_signalmash, '%d')AS da FROM mdr m )d;
SELECT @sql;
SET @sql = CONCAT('SELECT u.username, ' , @sql, ' 
from mdr m ',
 'join user u',
  'on u.iduser = m.iduser',
'group by u.username');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

what's wrong in this query?

error:-Incorrect parameters in the call to native function 'CONCAT'