The 261st prohibition is that one who has purchased a female Jewish servant is forbidden from selling her1 to another person.
The source of this prohibition is G‑d's statement2 (exalted be He), [By not marrying her] "he is considered to have broken faith with her, and he therefore does not have the right to sell her to anyone else."
The details of this mitzvah are fully explained in the beginning of tractate Kiddushin.3
