The gilded pipe organ (1714) by Johann David Sieber is the largest Baroque organ in Vienna. It was once played by the 17-year-old Joseph Haydn in 1749. Mozart's Requiem was performed for the first time in this church at a memorial service for the composer on 10 December 1791. As Mozart hadn't finished this work at the time of his death, only the existing part was performed. One of those who attended the "festive funerary honors" was theater director Emanuel Schikaneder whose libretto was used by Mozart for "The Magic Flute".