opera

"I had an idea that I would like to use to open a film. We are at Covent Garden or La Scala. Maria Callas is onstage. She is singing an aria, and her head is tilted upwards. She sees, in a box high up, a man approach another man who is seated there. He stabs him. She is just reaching a high note, and the high note turns into a scream. It is the highest note she has ever sung, and she receives a tremendous ovation ... "

... who said this?