To change the template of notification mail when an user is assigned

I want to change the email notification template when a user is assigned to in invoices module, where can i do this ?
i tried to look through in include/language/ en_us.notify_template.html but does not have the invoices there.

Can you copy-paste an example of what the email looks like? This will help me trace the place in the code where it is defined. Thanks

You can create your custom template and use workflow module to send the email notification to your users.
1 Create Email template for invoice module.
2 Create a workflow with the condition
when the assigned user changes you can set the action to send the mail to the assigned user.