he did what!!!! today the Miserere mei, Deus is well known but back then apparently at some time in history the piece became forbidden to be copied.
But the story is a bit more complicated than Mozart just copying it…
Miserere (Allegri) https://en.wikipedia.org/wiki/Miserere_(Allegri)
Mozart’s illegal copy of Allegri’s Miserere http://www.andrea-angelini.eu/mozart-miserere/
If it wasn’t for Mozart, this piece would have been forgotten forever http://www.classicfm.com/composers/mozart/guides/mozart-allegri-miserere/
That time Mozart pirated a forbidden piece of music from the Catholic Church from memory http://www.todayifoundout.com/index.php/2017/01/time-mozart-pirated-music-1700s/
Miserere mei, Deus Lyrics and English Translation https://www.thoughtco.com/miserere-mei-deus-lyrics-and-english-translation-723977