Positive commandment · Mitzvat Aseh #91 of 613
To burn the leftovers [from the sacrifices]
לשרוף הנותר
The verse · Vayikra 7:17
וְהַנּוֹתָר מִבְּשַׂר הַזָּבַח בַּיּוֹם הַשְּׁלִישִׁי בָּאֵשׁ יִשָּׂרֵף
“but that which remains of the flesh of the sacrifice on the third day shall be burnt with fire.”
The Rambam’s words
To burn the leftovers [from the sacrifices], as [Leviticus 7:17] states: "The leftovers from the flesh of the sacrifice shall be burnt with fire on the third day."
לשרוף הנותר, שנאמר: "והנותר, מבשר הזבח--ביום, השלישי, באש, יישרף" (ויקרא ז,יז).
Why · Sefer HaChinuch #143
Burning of the remnant (notar) of the sacrifices
“It is from the roots of the commandment [that it is] since the nature of all meat is (to spoil) [to become disqualified] by sitting out and coming to (putrification) [loss].”
Who & when
“And [it] is practiced at the time of the [Temple] by the males of the priesthood, as the service is for them.”
Where in the Mishneh Torah
Hilchot Sacrifices Rendered Unfit הלכות פסולי המוקדשין