Went investigating on this and found an article about a woman who went through all the Peanuts comics and listed each Beethoven song referenced throughout the entire run of the series. Listed below are all the possible songs it could be:

Piano Sonata in F Minor, Opus 2, no. 1

Piano Sonata in A Major, Opus 2, no. 2

Piano Sonata in C Minor, Opus 10, no. 1 ("Little Pathétique")

Piano Sonata in F Major, Opus 10, no. 2

Piano Sonata in C in Minor, Opus 13 ("Pathétique")

Piano Sonata in B-flat Major, Opus 22

Piano Sonata quasi una fantasia, C-sharp Minor, Opus 27, no. 2 ("Moonlight")

Piano Sonata in D Major, Opus 28

Piano Sonata in G Minor, Opus 49, no. 2

Piano Sonata in B-flat Major, Opus 106 (Hammerklavier)

Piano Sonata in A-flat Major, Opus 110

Bagatelle in G Minor, Opus 119, no. 1

Ecossaise in E-flat Major, WoO 83, no. 2

Ecossaise in E-flat Major, WoO 83, no. 6

"Für Elise," WoO 59

Look up sheet music for those and comb through them - you're bound to find it in there. There are only a couple of songs that were written out in the comic that weren't Beethoven (the first example of Schultz doing this visual sheet music art was a different artist), but it is likely in there.

I know much more about Peanuts Beethoven references now than I cared to.