Symphony No. 8 (Mahler)
Symphonies are usually pieces of music played just by an orchestra, but Beethoven started a tradition of having singing in symphonies: his Ninth Symphony is the famous one with the Ode to Joy in the last movement.  Mahler took up this idea of combining singing with symphony.  His Symphonies Nos 2, 3, and 4 all have singing in them.  The next three are just for orchestra, but in No. 8 he uses singing again.