"One of the most famous acts of the saints is an operation to replace the amputated ulcerous leg of one of the patients [with] the foot of a recently deceased Moor. This story was reflected in many works of art.More on Saints Cosmas and Damian.
This episode is described in the text of an incunabula from the life of the saints, which appeared in Augsburg in 1489...
When the patient woke up, the pain did not happen. He got up and ordered his servants to bring candles. He always told me what had happened to him. People ran to the coffin of the Moor and saw a cut off leg. They rejoiced at the miracle that was accomplished and thanked God and the saints of Cosmas and Damian with fervor. "
Top image: School of Castile and Leon, 'Saints Cosmas and Damian Healing a Christian with the Leg of a Dead Moor', 1460-1480. Both images via Marinni's LiveJournal, where there are additional depictions of the legend.