link between PO and Delivery

How do we get the Purchase order number from a delivery.

Say i have an Purchase order and from that i have created a delivery.

In my output for the delivery, i need the preceeding document number for the delivery which is the PO number.

How do i get the same. what table should i look in. I checked the VBFA table, but i am not able to get the same there...

is there any table with which we can get this info.

thanks and Regards,

Gauravjit.