The famous story here is that of Henry Beecher, usually considered discoverer of the placebo. From the Telegraph:

The power of the placebo first came to light during the Second World War. Morphine was in short supply in military field hospitals and an American anaesthetist called Henry Beecher, who was preparing to treat a soldier with terrible injuries, feared that without the drug the operation could induce a fatal heart attack. In desperation, one of the nurses injected the man with a harmless solution of saline. To Beecher’s surprise the patient settled down as if he had been given morphine and felt little pain during the operation.

Or for a more formal study, see Analgesic responses to morphine and placebo in individuals with postoperative pain