2 Chronicles 36:20-21

20He exiled to Babylon those who had escaped the sword and they became slaves to him and his sons until the reign of the kingdom of Persia—

21in fulfillment of the word of Adonai by the mouth of Jeremiah, until the land had paid back her Shabbat rests—for as long as it lay desolate—the Shabbat rest was kept till 70 years were complete.