When Beethoven praised Mozart's Die Zauberflöte, he emphasized the multitude of musical styles and genres to be found in the opera, ranging from folk tunes to arias to hymn-like textures. The most extraordinary stylistic and generic allusions occur during the ‘Song of the Armoured Men’ in Act 2. This movement owes its extraordinary character to a ‘baroque’ accompaniment and a Lutheran-hymn quotation, the source and meaning of which continue to be discussed in Mozart research. While scholars have often suggested that Mozart took the hymn melody from Johann Philipp Kirnberger's Die Kunst des reinen Satzes in der Musik (where it is quoted without its text), Reinhold Hammerstein argues that because the composer also appears to have known the hymn's text, he must have encountered the melody elsewhere. This article, based on a study of Mozart's sketches for Die Zauberflöte and a close reading of Kirnberger's writings, supports the thesis that Mozart borrowed the hymn melody – and significant details of his setting of it in the ‘Song of the Armoured Men’ – from Die Kunst des reinen Satzes in der Musik.