The Sixteen Miserere Mei Deus

from my quick web search, this music is written by Allegri, in setting of Psalm 51 and used only at special Mass in Sistine Chapel. It was forbidden to public for reproduction untill Mozart at age of 14, transcribed the whole music in 2 listening. Instead of excommunication, Mozart was praised by the pope and the ban was lifted for rest of the world to hear.