Jeremiah 34:13-14

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.