John Calvin Commentary Genesis 39:19

"And it came to pass, when his master heard the words of his wife, which she spake unto him, saying, After this manner did thy servant to me; that his wrath was kindled." — Genesis 39:19 (ASV)

When his master heard the words of his wife. Seeing that such a plausible appearance was given to the event, it is no wonder that jealousy, whose impulses are extremely strong and passionate, should have influenced Potiphar so much as to make him believe his wife's slanderous accusations.

Yet the carelessness with which he immediately threw a servant, whom he had found wise and honest, into prison, without investigating the matter, cannot be excused. He certainly should have been less under his wife's influence. And therefore, he received the just reward for his gullibility, by cherishing with honor a harlot in place of a wife, and by almost acting as a pimp.

This example is useful to all; yet husbands especially are taught that they must use prudence, lest they should be rashly swayed one way and another at the will of their wives. And truly, since we see everywhere that those who are too subservient to their wives are held up to ridicule, let us understand that the folly of these men is condemned by the just judgment of God, so that we may learn to pray for a spirit of seriousness and moderation.

There is no doubt that Moses expressly condemns Potiphar's rashness in becoming enraged against Joseph as soon as he heard his wife, and in unleashing his indignation just as if Joseph's guilt had been proven. For in this way all fairness is excluded, no just defense is allowed, and finally, the true and accurate investigation of the matter is utterly rejected.

But it may be asked, how could Potiphar's jealousy be aroused, since Moses previously said that he was an eunuch? The solution to this question is easy: in the East, they were accustomed to call not only those who actually were eunuchs by that name, but also satraps and nobles. Therefore, this name has the same meaning as if Moses had said that he was one of the chief men of the court.