Eine kleine Nachtmusik
From Memory Alpha, the free Star Trek reference
Eine kleine Nachtmusik was a piece of classical music written by Earth composer Wolfgang Amadeus Mozart in the late 18th century.
A crew member of the USS Enterprise-D fantasized playing the first movement ("Allegro") of Eine kleine Nachtmusik together with three other musicians in period clothing, when he was affected by being in a far away galaxy where thoughts became reality in 2364. (TNG: "Where No One Has Gone Before")
A string quartet consisting of Data (playing the violin), Miles O'Brien (playing the cello), and two more crewmembers of the Enterprise-D (playing the violin and viola) performed the first movement of Eine kleine Nachtmusik at the beginning of the year 2366. Captain Jean-Luc Picard later listened to an audio recording of the second movement ("Andante") that Doctor Beverly Crusher had made as he wasn't able to attend the concert. (TNG: "The Ensigns of Command")
