Negative commandment · Lo Ta’aseh #116 of 613
Not to allow the sacrificial portions of the Paschal sacrifice to become disqualified by remaining overnight
שלא להניח אימורי הפסח עד שיפסלו בלינה
The verse · Shemot 23:18
לֹא־תִזְבַּח עַל־חָמֵץ דַּם־זִבְחִי וְלֹא־יָלִין חֵלֶב־חַגִּי עַד־בֹּקֶר
“Thou shalt not offer the blood of my sacrifice with leavened bread; neither shall the fat of my sacrifice remain until the morning.”
The Rambam’s words
Not to allow the sacrificial portions of the Paschal sacrifice to become disqualified by remaining overnight, as [Exodus 23:18] states: "Do not allow the fat of My festive offering to remain until the morning."
שלא להניח אימורי הפסח עד שיפסלו בלינה, שנאמר "ולא ילין חלב חגי, עד בוקר" (שמות כג,יח).
Why · Sefer HaChinuch #90
To not leave the entrails of the Pesach sacrifice to stay overnight
“It is from the roots of the commandment [that it is] because it is the honor of a sacrifice to sacrifice it at its time that is set for it. And one who passes the set time appears like one who abandons [it] and throws the matter over his back; and he does not arouse himself and properly place his intentions upon the service.”
Who & when
“And [it] is practiced at the time of the Temple by male priests.”
Where in the Mishneh Torah
Hilchot Paschal Offering הלכות קרבן פסח