Mozart became famous at the age of 14 by transcribing Allegri's Miserere from memory after hearing it performed in the Sistine Chapel. It was a piece of music that had been composed about 150 years earlier, and which was not permitted to be written down or distributed, only to be performed once a year during Holy Week services in the Chapel.
https://en.wikipedia.org/wiki/Miserere_(Allegri)