Jeremiah 34:14
34:14 “Every seven years each of you must free any fellow Hebrews who have sold themselves to you. After they have served you for six years, you shall set them free.” 1 But your ancestors did not obey me or pay any attention to me.
1 sn Compare Deut 15:12-18 for the complete statement of this law. Here only the first part of it is cited.