Adam Clarke on Genesis 40:3

Where Joseph was bound - The place in which Joseph was now confined; this is what is implied in being bound; for, without doubt, he had his personal liberty. As the butler and. the baker were state criminals they were put in the same prison with Joseph, which we learn from the preceding chapter, Gen 39:20, was the king's prison. All the officers in the employment of the ancient kings of Egypt were, according to Diodorus Siculus, taken from the most illustrious families of the priesthood in the country; no slave or common person being ever permitted to serve in the presence of the king. As these persons, therefore, were of the most noble families, it is natural to expect they would be put, when accused, into the state prison.