Hi Sumit,
User exit: FORM USEREXIT_SAVE_DOCUMENT_PREPARE is used to introduced checks before the document is saved. So I suppose that this means that if the document is not saved, then there is no sales order number.
I am not sure what are you doing or need to do. If you need to see the sales order number, maybe you will need to use other exist, like FORM USEREXIT_SAVE_DOCUMENT, in which you see the sales order number.
Also, I would recommed to move this thread to a different space. It is more convininet to move it to SAP SD:
SAP ERP Sales and Distribution (SAP SD)
Kind Regards,
Mariano