This is one of the last compositions of Beethoven. He wrote this slow movement of his 15th quartet as thanksviging for having survived a near-fatal illness. It's a deeply moving piece and widely regarded as one of his greatest masterpieces (as the rest of the late quartets). I recommend listening to this piece (and preferably the entire quartet, of course) in complete silence; if one listens carefully, one cannot help but to share the existential experience felt as communicated by the master.
This is a legendary recording by the Julliard Quartet.
Ludwig van Beethoven : Quartet #15 Molto Adagio (Heiliger Dankgesang eines Genesenen an die Gottheit, in der lydischen Tonart).
Interpreter: Julliard Quartet
Album: Beethoven : Complete String Quartets.