Eine Kleine Nachtmusik (A Little Night Music) is a suite of movements for string orchestra by Wolfgang Amadeus Mozart. It is his Serenade No. 13 in G major and catalogued as K.525. Its charm and rich melodies have long made it a favourite among audiences and from time to time it can be heard on television, in films or accompanying adverts. The opening movement featured here is full of boisterous good humour. This particular arrangement is for clarinet in B-flat and piano, and you can print the individual parts here: Part for B-flat Clarinet and Piano Part. Alternative arrangements for a range of other instruments plus piano accompaniment can be accessed using the links on the left hand menu, or for comparison here is Mozart's original version for String Orchestra.
The image below is the first page of the sheet music, and here you can download the full score for Mozart's Eine Kleine Nachtmusik (1st Movement) for clarinet in B-flat and piano in PDF format.