Rashi on Exodus 21:4/רש"י על שמות כ״א:ד׳

1אם אדניו יתן לו אשה IF HIS LORD HAS GIVEN HIM A WIFE — From this we learn that if he has already an Israelite wife the master has the right to give him a Canaanitish handmaid with the object of raising slaves. Or perhaps this is not so, but Scripture by אשה is speaking about an Israelite woman?! Scripture, however, states: “the wife and her children shall be the master’s”, consequently the text can only be speaking of a Canaanitish woman, for a Hebrew maidservant goes free at the end of six years just as a Hebrew man-servant does — yea, even before the termination of six years she goes free if she shows symptoms of incipient puberty (cf. Rashi v. 7) — for it is said, (Deuteronomy 15:12) “[and if] thy brother, an Hebrew man, or an Hebrew woman, [be sold unto thee, and serve thee six years, then in the seventh year thou shalt let him go free from thee]”, which statement teaches you that a Hebrew woman also goes free after six years’ service (Mekhilta; Kiddushin 14b).