The 254th prohibition is that we are forbidden from returning to his master a servant who has fled to Israel. This applies even if his master is Jewish — since he has run to Israel from outside Israel, he should not be returned1 to him. Rather, he must be freed, with his remaining value written down as a debt [owed by the former servant to his former master].
The source of this prohibition is G‑d's statement2 (exalted be He), "You must not turn him back over to his master."
It is explained in the 4th chapter of tractate Gittin3 that this verse refers to a servant who has run to Israel from outside Israel; that a document is written regarding his monetary value; that a get cherus (liberation document) is written for him; and that since he has come to dwell in the pure land which was chosen for the holy people, he shall never be returned to servitude. The details of this mitzvah are explained there.
