This is the 7th fugue (in E flat) from book 2 of Johann Sebastian Bach's Well Tempered Clavier, also known as the 48 Preludes and Fugues. Bach's Preludes and Fugues come in matched pairs in the same key in this collection, and are frequently (though not always) played as a pair in the order Prelude first and then Fugure. Here is the corresponding Prelude no.7 for this Fugue, also in the key of 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.