Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 254
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 254, with English translation by Maimonides (Rambam).
שלא להחזיר עבד שברח לארץ ישראל לאדוניו שבחוצה לארץ, שנאמר "לֹא תַסְגִּיר עֶבֶד אֶל אֲדוֹנָיו" (דברים כג, טז).
Not to return a slave who fled to Eretz Yisrael to his master [who lives] in the Diaspora, as [Deuteronomy 23:16] states: "Do not deliver a servant to his master."