Mitzvah 90) – To not allow the burnt-portions of the Pesach offering to become disqualified by leaving them overnight
Torah Portion: Mishpatim
…nor may the fat of My festive-offering remain overnight until morning(Shemos 23:18).
It is a negative commandment to not let the ’eimurim (the inner organs) of the Pesach offering [that are intended to be burnt on the Altar] remain until morning — they become disqualified by this delay, and are called nosar (“left-over”) — as is written, the fat of My festive-offering may not remain overnight until morning (Shemos 23:18). This law also applies to other ’eimurim of other offerings as well.
Among the roots of the mitzvah is the idea that part of honoring the offering is to offer it during the time that was set for it. But a person who does not do this demonstrates that it is not that important to him and that is why he did not arouse himself properly and aim to do the service properly. It is because of this that offerings become disqualified [by such delays].
This mitzvah applies when the Holy Temple stands, to kohanim, for it is their responsibility to burn the ’eimurim.