Parashas Behar · Aliyah 7 · Passuk 25:55
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra / Kiddushin 22b.
English: “For to Me the children of Israel are servants.” They are My servants — and not servants to servants.
Classical derasha (Sifra / Bavli Kiddushin).
The ultimate rationale for all the laws of the eved ivri: Israel belongs to God alone; therefore permanent enslavement to another human is forbidden.