thank you... in which add on?

Use below python code in your add-on to send a message to user.

from openerp.osv import fields, osv
from openerp.tools.translate import _
class project_task(osv.osv):
    _name = 'project.task'
    def create(self, cr, uid, vals, context=None):        
        new_id = super(project_task, self).create(cr, uid, vals, context=context)
        self.message_post(cr, uid, [new_id], body=_("Task Created"), subtype='mail.mt_comment', context=context)
        return new_id