Parashas Behar · Aliyah 7 · Passuk 26:2
Chabad.org — English translation + Rashi
Torah Temimah cites Sifra (Torat Kohanim).
English: “My Sabbaths you shall observe.” Directed at one who has been sold to a non-Jew, that he should not say: Since my master works on Shabbos, I will do likewise.
Classical derasha (Sifra).
Even under non-Jewish ownership, the Hebrew servant remains fully obligated in Shabbos observance.