Parashas Behar · Aliyah 6 · Passuk 25:43
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra / Kiddushin 22a.
English: “You shall not rule over him with rigor.” That you may not say to him “Warm this cup for me” when you have no need of it.
Classical derasha (Sifra / Bavli Kiddushin).
Rigorous rule includes assigning unnecessary labor solely to assert dominance; the warning “fear your God” is attached because intent is known only to the heart.