Jeremiah 34:7-9

Amplified Bible

⁷ when the army of the king of Babylon was fighting against Jerusalem and against all the remaining cities of Judah, against Lachish and Azekah, for these were the [only] fortified cities among the cities of Judah.

⁸ The word came to Jeremiah from the Lord after King Zedekiah had made a covenant (solemn pledge) with all the [Hebrew] people who were [slaves] in Jerusalem to proclaim liberty to them: ⁹ that every man should let his Hebrew slaves, male and female, go free, so that no one should make a slave of a Jew, his brother.