If thou buy an Hebrew servant, six years he shall serve: and in the seventh he shall go out free for nothing. (21:2)

--Deuteronomy--

[And] if thy brother, an Hebrew man, or an Hebrew woman, be sold unto thee, and serve thee six years; then in the seventh year thou shalt let him go free from thee. (15:12)

--Jeremiah--

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. (34:14)