Parashas Behar · Aliyah 7 · Passuk 25:53
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra / Kiddushin 22a.
English: “He shall not rule over him with rigor before your eyes.” This teaches that you are commanded concerning him.
Classical derasha (Sifra / Bavli Kiddushin).
Even when a Hebrew servant is owned by a non-Jew, fellow Jews are obligated to ensure he is not treated with rigor; the duty is public and enforceable.