Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 222
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 222, with English translation by Maimonides (Rambam).
לגרש בשטר, שנאמר: "וכתב לה ספר כריתות ונתן בידה" (דברים כד,א; דברים כד,ג).
To divorce with a get, as [Deuteronomy 24:1] states: "And he shall write a bill of divorce for her and place it in her hand."