Someone lent me a copy of "The Friendship Book of Francis Gay - 1980".
The entry for July 3 reads:

  "The Italian composer Pietro Mascagni was sitting in his study one
day when a street musician stopped outside and began to play one of
Mascagni's pieces on his hand-cranked barrel organ.  He was turning
the handle too quickly so that the tempo was faster than it should have
been.  Mascagni put up with it for a few minutes, then he went outside,
grabbed the handle, and played the piece at its proper tempo.  He then
returned to his study.

  "Next day he was amused to see the street musician displaying a sign
which read, 'Pupil of the celebrated Mascagni'!"

Mark Williamson