Jamieson, Fausset and Brown on Genesis 39:21-23

21-23. The Lord ... gave him favour in the sight of the keeper of the prison, &c.--It is highly probable, from the situation of this prison (Ge 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, as well as with all the high integrity of his character. That may partly account for his showing so much kindness and confidence to his prisoner. But there was a higher influence at work; for "the Lord was with Joseph, and that which he did, the Lord made it to prosper."