Genesis 37:36

36.And the Midianites sold him into Egypt It was a sad spectacle, that Joseph should be thus driven from one hand to another. For it added no small indignity to his former suffering, that he is set to sale as a slave. The Lord, however, ceased not to care for him. He even suffered him to be transferred from hand to hand, in order that, at length, it might indeed appear, that he had come, by celestial guidance, to that very dominion which had been promised him in his dreams. Potiphar is called a eunuch, not because he was one really; but because, among the Orientals, it was usual to denote the satraps and princes of the court by that name. The Hebrews are not agreed respecting the dignity which Moses ascribes to him; for some explain it as the “chief of the slaughterers,” (136) whom the Greek interpreters follow. But I rather agree with others, who say that he was “the prefect of the soldiers;” not that he had the command of the whole army, but because he had the royal troops under his hand and authority: such are now the captains of the guard, if you join with it another office which the prefects of the prison exercise. For this may be gathered from Genesis 39:1 (137)