Documents describing Mozart's transcription of the Allegri Miserere
In April 1770, the 14-year-old Wolfgang Amadeus Mozart attended performances of the Miserere by Gregorio Allegri in the Sistine Chapel in Rome and made a transcription of the work from memory. The episode is often related in biographies of the composer. The following are two original source documents recounting the episode.
Letter from Leopold Mozart to his wife
[Sent from Rome, dated April 14, 1770. Only parts of the letter relevant to the transcription episode are given here.]
We arrived here safely on the 11th at noon. I could have been more easily persuaded to return to Salzburg than to proceed to Rome, for we had to travel for five days from Florence to Rome in the most horrible rain and cold wind. I am told here that they have had constant rain for four months and indeed we had a taste of it, as we went on Wednesday and Thursday in fine weather to Saint Peter's and to the Sistine Chapel to hear the Miserere during the mass, and on our way home were surprised by such a frightful downpour that our cloaks have never yet been so drenched as they then were...
You have often heard of the famous Miserere in Rome, which is so greatly prized that the performers in the chapel are forbidden on pain of excommunication to take away a single part of it, to copy it or to give it to anyone. But we have it already. Wolfgang has written it down and we would have sent it to Salzburg in this letter, if it were not necessary for us to be there to perform it. But the manner of performance contributes more to its effect than the composition itself. So we shall bring it home with us. Moreover, as it is one of the secrets of Rome, we do not wish to let it fall into other hands, ut non incurramus mediate vel immediate in censuram Ecclesiae.
--translation by Emily Anderson, in her The Letters of Mozart and his Family; London: Macmillan, 1938.
Reminiscences of Marianne von Berchtold, née Mozart
[Marianne von Berchtold was Mozart's elder sister, called "Nannerl" in the family. In spring of 1792, not long after Mozart's death, she wrote down her memories of her brother to assist a biographer. Nannerl was not present in Rome at the time of the event related, but presumably received a full report from Leopold and Wolfgang on their return to Salzburg.]
...they traveled on the 15th March 1770 to Parma, Bologna, Florence, [on] to Rome, where they arrived during Holy Week. On Wednesday afternoon they accordingly went at once to the Sistine Chapel, to hear the famous Miserere. And as according to tradition it was forbidden under ban of excommunication to make a copy of it from the papal music, the son undertook to hear it and then copy it out. And so it came about that when he came home, he wrote it out, the next day he went back again, holding his copy in his hat, to see whether he had got it right or not. But a different Miserere was sung. However, on Good Friday the first was repeated again. After he had returned home he made a correction here and there, then it was ready. It soon became known in Rome, [and] he had to sing it at the clavier at a concert. The castrato Christofori, who sang it in the chapel, was present.
--from Otto Erich Deutsch, Mozart: A Documentary Biography, translated by Eric Blom, Peter Branscombe, and Jeremy Noble. Stanford: Stanford University Press, 1965.