Its from The Book "Bring on the empty Horses" by David Niven
David Niven  and Errol Flynn were filming The Charge of the Light Brigade for a director, Michael Curtiz, 'whose Hungarian-orientated English was a joy to us all'. High on the rostrum he decided the moment had come to order the arrival on the scene of a hundred riderless chargers. "Okay," he yelled into a megaphone, "Bring on the empty horses!" '