A solemn, slow march … introduces the assembly of the priests [in The Magic Flute] in the most appropriate manner. It is said that in answer to the accusation of a friend that he had stolen this march from Gluck’s “Alceste” (Act I., sc. 3), Mozart laughingly replied that that was impossible, as it still stood there.
(Otto Jahn, Life of Mozart, vol. 3, trans. Pauline D. Townsend (London: Novello, Ewer & Co., 1882), p. 323.)