Genesis 40:2-3

And Pharaoh was wroth against two of his officers, against the chief of the butlers, and against the chief of the bakers.

Pharaoh ... put them in ward ... Whatever was their crime (and it has been supposed, from the nature of their employments, together with Oriental tradition, that it consisted in an attempt to poison the king), they were committed-until their case could be investigated-to the custody of the captain of the guard - i:e., Potiphar, in an outer part of whose house the royal prison was situated.