Jeremiah 34:12 (KJV)

Now when all the princes, and all the people, which had entered into the covenant, heard that every one should let his manservant, and every one his maidservant, go free, that none should serve themselves of them any more, then they obeyed, and let them go.

But afterward 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.

Therefore the word of the Lord came to Jeremiah from the Lord, saying,

Thus saith the Lord, 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 bondmen, saying,

At the end of seven years let ye go every man his brother an Hebrew, which hath been sold unto thee; and when he hath served thee six years, thou shalt let him go free from thee: but your fathers hearkened not unto me, neither inclined their ear.