Positive commandment · Mitzvat Aseh #222 of 613
To divorce with a get
לגרש בשטר
The verse · Devarim 24:1
כִּי־יִקַּח אִישׁ אִשָּׁה וּבְעָלָהּ וְהָיָה אִם־לֹא תִמְצָא־חֵן בְּעֵינָיו כִּי־מָצָא בָהּ עֶרְוַת דָּבָר וְכָתַב לָהּ סֵפֶר כְּרִיתֻת וְנָתַן בְּיָדָהּ וְשִׁלְּחָהּ מִבֵּיתוֹ
“When a man has taken a wife, and married her, and it come to pass that she find no favour in his eyes, because he has found some unseemliness in her: then let him write her a bill of divorce, and give it in her hand, and send her out of his house.”
The Rambam’s words
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."
לגרש בשטר, שנאמר: "וכתב לה ספר כריתות ונתן בידה" (דברים כד,א; דברים כד,ג).
Why · Sefer HaChinuch #579
That one who wants to divorce his wife divorce her with a bill (get)
“It is from the roots of the commandment [that] since woman was created to help man and she is like a beloved vessel for him — and like the matter that they, may their memory be blessed, said (Sanhedrin 22b), “A woman only makes a covenant with the one who makes her into a vessel” — since that is the case, it was from His will, blessed be He, that anytime his soul be sick of this vessel, that he should put her out of his house.”
Who & when
“And this commandment of divorce bills is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Divorce הלכות גירושין