I assume you heard the Siegfried Idyll, which is a stand-alone piece and is available in a number of recordings. Wagner did incorporate some of the music into the opera Siegfried, from Der Ring des Nibelungen, but that opera is four hours long and the portion incorporated rather miniscule.
I think there are swome instrumental pieces associated with Wagner's 15 hour long musical drama called Der Ring des Nibelungen. This work is so awesome i have half a dozen sets on video and probably a dozen or more on CD. I have been listing to it over and over since mid August and still loving it!