The 19-year-old singer was unable to try a series of 26 postures because the heat became unbearable for him, reported Contactmusic.
"Harry couldn't handle the heat. The class lasts 90 minutes and is pretty intense but the girls in the class were ribbing him for having to sit down during the exercises," a source said.
"He took all the winding up in good spirits and said afterwards it was the 'toughest session' he'd ever experienced. The girls thought he was a real gent," the source added.