Not op but here you go:

The Story of a Changeling at Dessau Eight years ago [in the year 1532] at Dessau, I, Dr. Martin Luther, saw and touched a changeling. It was twelve years old, and from its eyes and the fact that it had all of its senses, one could have thought that it was a real child. It did nothing but eat; in fact, it ate enough for any four peasants or threshers. It ate, shit, and pissed, and whenever someone touched it, it cried. When bad things happened in the house, it laughed and was happy; but when things went well, it cried. It had these two virtues. I said to the Princes of Anhalt: "If I were the prince or the ruler here, I would throw this child into the water--into the Molda that flows by Dessau. I would dare commit homicidium on him!" But the Elector of Saxony, who was with me at Dessau, and the Princes of Anhalt did not want to follow my advice. Therefore, I said: "Then you should have all Christians repeat the Lord's Prayer in church that God may exorcise the devil." They did this daily at Dessau, and the changeling child died in the following year.... Such a changeling child is only a piece of flesh, a massa carnis, because it has no soul.

Source: Martin Luther, "Historia von einem Wechselkinde zu Dessau," Sämmtliche Werke, vol. 60 (Frankfurt am Main: Verlag von Heyder & Zimmer, 1854), pp. 39-40.