פירוש על קידושין 31:5

A non-Jewish slave can be acquired through hazakah. Hazakah, which means presumption of ownership, is achieved when a slave performs a labor for a master that would be typical only of a slave. So why not say that just as slaves are acquired by hazakah, so too are Jewish female slaves?
The answer is that the rabbis read Leviticus 25:46 as limiting acquisition by hazakah to non-Jewish slaves.