Genesis 39:20

And Joseph's master took counsel of the priests, who ____ put him not to death, but delivered him into the house of the bound, where the king's prisoners were bound; and he was there in the house of the bound.

AND HE PUT HIM INTO PRISON, THE PLACE WHERE THE KING’s PRISONERS WERE BOUND. Rabbi Abraham ibn Ezra says that the verse itself explains that a beth haso’ar (prison) is “a place where the king’s prisoners were bound.” The reason this is stated in the verse itself is that beth haso’ar is an Egyptian word, for it is the style of Scripture to explain foreign words just as, they cast pur, that is the lot.
This interpretation is of no significance. Rather, And he put him into the prison, means that he put him into a certain prison recognized as the royal prison, which was the place where the king’s prisoners were bound. The sense of the verse is thus to state that this was the cause of the butler and the baker being imprisoned with him.
It is possible that the term, “the king’s prisoners,” means his servants and attendants who have sinned against him in matters of state, as other prisoners of the people sentenced by judges and officers were placed in another prison house. Scripture relates that they placed Joseph in the king’s prison because of his master’s love for Joseph, all of which was caused by G-d.
Linguists. explain sohar as an arched chamber, similar in expression to, agan hasohar (a round goblet). In my opinion it is an underground house having a small opening above ground, through which the prisoners are lowered and from which they have light. The word sohar is thus derived from the word sihara (light) in Aramaic, just as in Hebrew, Scripture says; A transparency (‘tzohar’) shalt thou make to the ark, the word tzohar being derived from tzaharayim (mid-day — when the light reaches its zenith). The difference between tzohar and sohar is that tzohar connotes an abundance of light, while sohar connotes minimal light.