There is no regulatory requirement to return the original recorded mortgage after a loan has been paid in full to the borrower. The original note with a paid stamp and the original recorded discharge of mortgage (or instructions to the borrower that they should record it themselves) does give the borrower the documentation they would need in the even they would need to show that the mortgage loan had been paid and the property was no longer encumbered.
However, there may be state law requirements in this area, so check with your institution's legal counsel. Also, be prepared to respond to those borrowers that are insistent on getting their original mortgage.