I want to Prevent sales users (profile: Sales Rep) from modifying the 'Discount' field in opportunity record when the 'Sales Stage' has reached Negotiation/Review. But I would like sales managers (profile: Sales Mgr) to be able to edit the opportuntiy for any changes.

I believe it is possible through Validation Rule on Opportunity object but not able to come up with the right formula. Please let me help structure the VR.
Also can I achieve this using Field Level Security or Workflow rule ??