I hate to be a spoil-sport, but the fact that the c-d-f-e motif occurs in
Mozart's first symphony (E flat, K 16) has been well known for many years.
The motif itself comes from plainchant, although I can't recall right now
from where exactly - I think it's one of the Gregorian Gloria movements.

("Well known" is of course a relative notion.)

Other plainchant snippet, the "tonus peregrinus", can be found in Mozart's
music:  in the Requiem (Introitus, on the words "Te decet Hymnus") and in
"Betulia liberata" (the final chorus "Lodi al gran Dio").

-Margaret Mikulska