Jeremiah 34:12

All the princes and all the people obeyed, who had entered into the covenant, that everyone should let his male servant, and everyone his female servant, go free, that none should make bondservants of them any more; they obeyed, and let them go:

but afterwards they turned, and caused the servants and the handmaids, whom they had let go free, to return, and brought them into subjection for servants and for handmaids.

Thus says Yahweh, the God of Israel: I made a covenant with your fathers in the day that I brought them forth out of the land of Egypt, out of the house of bondage, saying,

At the end of seven years you shall let go every man his brother who is a Hebrew, who has been sold to you, and has served you six years, you shall let him go free from you: but your fathers didn't listen to me, neither inclined their ear.