Справочник к Шмот 21:16

These men were executed—the legend continues to relate—when they were reading the Torah to the empreror and they came to the law which states that a person who steals a man and enslaves him, is to be put to death. (Ex. 21:16.) The emperor then said: “Where are your forefathers, the sons of Jacob, who sold their brother into slavery? If I had them here I would immediately have them put to death. But since they are not here you will have to atone for their crime.”