A korban todah (thanksgiving sacrifice) had to be eaten on the day it was offered; anything that remained the following day was called nosar (leftover) and was prohibited. (As we will see in the next mitzvah, nosar must be burned.)
The penalty for eating nosar was kareis (spiritual excision). In Mitzvah #144, we will discuss piggul, a sacrifice that has been made disgusting to God through improper intentions. The Torah also uses the term piggul to describe nosar (see Leviticus 7:18 and 19:7). One might think that if a person eats nosar, the sacrifice from which it was left over is invalidated. Rashi (on 7:18) clarifies that this is not the case. Once accepted, a sacrifice cannot be retroactively overturned.
The reasons underlying the prohibition on nosar will be discussed in the next mitzvah.
This mitzvah applies to male kohanim in Temple times. It is discussed in the Talmud in tractates Pesachim (27b), Temurah (34a) and Chulin (90b, 125a). It is codified in the Mishneh Torah in the eighteenth and nineteenth chapters of Hilchos Pesulei HaMukdashin. It is #120 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.