Negative commandment · Lo Ta’aseh #112 of 613
Not to sever the head of a bird [brought as] a sin offering
שלא יבדיל ראש חטאת העוף
The verse · Vayikra 5:8
וְהֵבִיא אֹתָם אֶל־הַכֹּהֵן וְהִקְרִיב אֶת־אֲשֶׁר לַחַטָּאת רִאשׁוֹנָה וּמָלַק אֶת־רֹאשׁוֹ מִמּוּל עׇרְפּוֹ וְלֹא יַבְדִּיל
“And he shall bring them to the priest, who shall offer that which is for the sin offering first, and wring off its head from its neck, but shall not divide it asunder:”
The Rambam’s words
Not to sever the head of a bird [brought as] a sin offering, as [Leviticus 5:8] states: "He shall pinch off its head...[without separating it]."
שלא יבדיל ראש חטאת העוף, שנאמר "ומלק את ראשו ממול עורפו [ולא יבדיל]" (ויקרא ה,ח).
Who & when
“And it is practiced at the time of the [Temple] by priests and by every person, as anyone who severs a foul sin-offering is lashed.”
Where in the Mishneh Torah
Hilchot Sacrificial Procedure הלכות מעשה הקרבנות