The 235th mitzvah is that we are commanded regarding the treatment of Canaanite servants:1 that we should have them serve us forever, going free only in [a case where the master struck them and caused them to lose] a tooth or [use of] an eye. The same applies to any exposed organ which does not grow back, as explained in the Oral Tradition.2
The source of this commandment are G‑d's statement3 (exalted be He), "You shall have them serve you forever" and,4 "If a person strikes [his male or female servant in the eye...the tooth...he shall set the servant free...in compensation for his eye...in compensation for his tooth]."
In the words of the Talmudic tractate Gittin5: "Anyone who frees his servant transgresses a positive commandment, as it is written, 'You shall have them serve you forever.' " The words of the Written Torah itself show that he must be freed upon loss of a tooth or an eye.
The details of this mitzvah are completely explained in tractates Kiddushin6 and Gittin.
(Others see the prohibition of not freeing a Canaanite slave as a concession, rather than an actual prohibition., If one did free his slave, the slave became fully Jewish. The Talmud records a rabbi freeing his slave in order to count as the tenth man i a minyan-J.S.)