Exodus 21:2 If thou buy an Hebrew servant, six years he shall serve: and in the seventh he shall go out free for nothing. - Free Bible Online

      2-6. If thou buy an Hebrew servant--Every Israelite was free-born; but slavery was permitted under certain restrictions. An Hebrew might be made a slave through poverty, debt, or crime; but at the end of six years he was entitled to freedom, and his wife, if she had voluntarily shared his state of bondage, also obtained release. Should he, however, have married a female slave, she and the children, after the husband's liberation, remained the master's property; and if, through attachment to his family, the Hebrew chose to forfeit his privilege and abide as he was, a formal process was gone through in a public court, and a brand of servitude stamped on his ear (Ps 40:6) for life, or at least till the Jubilee (De 15:17).

JFB.