They are linked if you selected that estimate when you invoiced.
Open the estimate and use the History icon. If this is linked to an invoice, you can see it and Go To from here. Open the Invoice and do the same thing = History icon, and Go To what is linked, from here.
If you use Progress invoicing, there also is a little Progress Chart at the top of the Invoice, which shows the details from the Estimate and proves they are linked.