Hi Mahi,
Yes, It is Customer invoices! Not incoming invoices !
In general the reference will follow!
Sales document to delivery
Delivery to customer invoice - this is your requirement to get the reference delivery number to create the invoice!
You need to retrieve all the delivery information details and need to pass it to the FM "GN_INVOICE_CREATE".
Try to check the below FM's also for more information
RV_Invoice_create & RV_INVOICE_DOCUMENT_ADD