Beethoven's Piano Sonata No.8 in C minor is known as the "Sonata Pathétique". The slow movement of this sonata is the most famous part and, because of its lyrical melody and steady accompaniment, it is readily arranged for other instrument combinations. This version has been arranged by Artur Akshelyan for Viola and Piano, and you can download the piano and viola sheet music parts (and midi and mp3 versions) using the links in the left-hand menu.
For more classical music for Viola and Piano we suggest "Time Pieces" which comes in 2 volumes. Volume 1 is available at musicroom.com and at sheetmusicplus.com, while volume 2 is available at these alternative links from musicroom.com and at sheetmusicplus.com.