Positive commandment · Mitzvat Aseh #82 of 613
To decapitate a firstling donkey [which is not redeemed]
לערוף פטר חמור
The verse · Shemot 34:20
וּפֶטֶר חֲמוֹר תִּפְדֶּה בְשֶׂה וְאִם־לֹא תִפְדֶּה וַעֲרַפְתּוֹ כֹּל בְּכוֹר בָּנֶיךָ תִּפְדֶּה וְלֹא־יֵרָאוּ פָנַי רֵיקָם
“But the firstling of an ass thou shalt redeem with a lamb: and if thou redeem it not, then shalt thou break its neck. All the firstborn of thy sons thou shalt redeem. And none shall appear before me empty.”
The Rambam’s words
To decapitate a firstling donkey [which is not redeemed], as [Exodus 34:20] states: "If you do not redeem it, you must decapitate it."
לערוף פטר חמור, שנאמר: "ואם לא תפדה וערפתו" (שמות יג,יג; שמות לד,כ).
Who & when
“The laws of the commandment — that which they said (Bekhorot 13a) that the commandment of redemption takes precedence over beheading; that it is forbidden to benefit from it if it dies before the redemption and that it should [then] be buried; so [too,] that he should kill it with a kofitz (a large knife), and not with something else; and the rest of its details — are elucidated in Bekhorot. (See Tur, Yoreh Deah 221.) And it is practiced like the redemption regarding everything.”
Where in the Mishneh Torah
Hilchot First Fruits and other Gifts to Priests Outside the Sanctuary הלכות ביכורים ושאר מתנות כהונה שבגבולין