Beethoven's Symphony No. 9, Fourth Movement - An Die Freud.
^^ Full lyrics and English translation are on the Wikipedia page linked above ^^
And here's a video with lyrics and English translation following the symphonic version:
This track and much of the synthesizer music on that soundtrack was arranged and performed by Wendy Carlos, who is probably best known for "Switched-On Bach". She went on to work with Kubrick again, composing much of the soundtrack for "The Shining".