Genesis 39:21-23

But the LORD was with Joseph, and shewed him mercy, and gave him favour in the sight of the keeper of the prison.

The Lord ... gave him favour ... It is highly probable, from the situation of this prison (Genesis 40:3), that "the keeper" might have been previously acquainted with Joseph, and have had access to know his innocence of the crime laid to his charge, and with all the high integrity of his character. Delitzsch, in attempting to account for the lightness of the punishment, considering the severity of the Egyptian law as to adultery, which was punished with one thousand blows, and rape on a free woman with a greater number says, 'It is possible that Potiphar was not fully convinced of his wife's chastity, and therefore did not place unlimited credence in what she said.' That may partly account for his showing so much kindness and confidence to his prisoner. But there was a higher influence at work; because "the Lord was with Joseph, and that which he did, the Lord made it to prosper."