Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 233
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 233, with English translation by Maimonides (Rambam).
לייעד אמה עברייה, שנאמר: "אשר לו יעדה" (שמות כא,ח), "ואם לבנו, ייעדנה" (שמות כא,ט).
To marry a Hebrew maidservant, as [Exodus 21:8] states: "Who has designated her for himself... and she will be redeemed."