There are a few different works called Ständchen or Serenade. This one is no.4 from a collection of songs D.957 published shortly after the death of their composer Franz Schubert and called Schwanengesang (or Swan Song) by the publisher. The song is a popular one and readily lends itself to different arrangements. The music illustrated on this page has been adapted for Piano Solo. You can download the sheet music, midi and mp3 versions using the links in the left-hand menu. Below is a video of this arrangement being played, and below that you can play the mp3 file and see the sheet music before downloading. For singers here is the Original Song and for guitar players here is a Guitar arrangement by Mertz, and we also have an arrangement for Flute and Piano with alternative parts to allow other instruments to substitute for the flute.
Here is a video of Schubert's Ständchen (Serenade) for piano:
The image below is the first page of the sheet music, and here you can download the full sheet music for Schubert's Ständchen (Serenade) for piano in PDF format.