Just assert your copyright in the program code (if supplied) in the executable and in any documentation - assuming that you have not relinquished the copyright

Just assert your copyright in the program code (if supplied) in the executable and in any documentation - assuming that you have not relinquished the copyright in the contract you have with your customer. You do not need an special clauses - just a standard copyright notice. If your customer wants a written license - then you might ensure that this includes the words "non exclusive".