Genesis 39:20

And Joseph's master took him, and put him into the prison,
&c.] Which was in or adjoining to his house, ( Genesis 40:3 ) ; of himself he had power to do this, as the captain of the guard; and as he was the chief of the executioners, as some take his office to be, it is much he did not in his passion deliver him up into their hands to put him to death at once; but it may be through the great respect he had had for Joseph, which was not wholly extinguished by this impeachment of him; and especially if he heard Joseph's apology for himself before he committed him, his passion might subside a little, though for the credit of his wife he might take this step; or however things were so overruled by the providence of God, who has the hearts of all men in his hands, that he should do what he did. The word for "prison" has the signification of roundness, and may be rendered the "round house" ^F20, or "round tower"; and some Jewish writers, as Mercer observes, take it to be in this form, that it was made under ground, and at the top of it was an hole which let in light, and at which they went into it. Aben Ezra is at a loss to say whether it is an Hebrew or Egyptian word, and inclines to the latter, because he thinks it is explained in the next clause: a place where the king's prisoners [were] bound;
such as were guilty of high treason, or however of high crimes and misdemeanours against him; and so was a prison in which men were strictly kept and used hardly, as was Joseph at first, as appears from ( Psalms 105:18 ) : and he was there in the prison;
he continued there, some of the Jewish writers say ^F21 ten years, others twelve ^F23; and so long he must be, if he was but one year in Potiphar's house; for there were thirteen years between his being sold into Egypt, and his appearance before Pharaoh; he was seventeen when he was sold, and he was thirty when he stood before Pharaoh, being took out of prison, see ( Genesis 37:2 ) ( 41:46 ) ; but it is more likely that he was a longer time in Potiphar's house, and a lesser time in prison.