This prelude in the key of E flat major in the 7th prelude from book 2 of Johann Sebastian Bach's Well Tempered Clavier, also known as the 48 Preludes and Fugues. It features on the soundtrack to the film "Milk" in an arrangement for unaccompanied voices performed by The Swingle Singers. Bach's Preludes and Fugues come in matched pairs in the same key, and are often (though not always) played as a pair in the order Prelude first and then Fugure. Here is the corresponding Fugue for this Prelude also in E-flat. A must for any serious student of piano, Book 1 and Book 2 are available from Sheet Music Plus.
Here is a video of this music, or scroll down the page to see the sheet music.