Where did the expression "swan song" originate?

A swan song is the final performance of an actor, singer, composer, poet, or the like. According to folklore, swans sing most beautifully before they die, and hence this phrase came to be used to describe someone who was leaving in style. The phrase is of ancient origin and is found in the works of Plato, Aristotle, Euripides, and Cicero. Later Shakespeare used it in several plays.