Jeremiah 34:13-14

13“This is what the Lord God of Israel says: I put a condition on the promise  I made to your ancestors when I brought them from Egypt, where they were slaves. I said,

14‘Every seven years each of you must free any Hebrews who sold themselves to you. When they have served you for six years, you must set them free.’ But your ancestors refused to obey me or listen to me.