Rashi on Leviticus 6:10/רש"י על ויקרא ו׳:י׳

1לא תאפה חמץ חלקם IT SHALL NOT BE BAKEN AS ANYTHING LEAVENED FOR THEIR PORTION — The remains of the meal-offering which become the portion of the priests, are also forbidden to be baked in the form of leavened (Menachot 55a).

2כחטאת וכאשם [IT IS MOST HOLY] AS THE SIN OFFERING OR THE GUILT OFFERING — the meal-offering of the sinner (v. 11) is as the sin-offering, therefore if he (the priest) takes the fistful out of it not as such (not having in mind that offering) it is invalid as is the sin-offering under such circumstances (cf. Rashi on 5:12); a free-will meal-offering (i.e. any of the other meal-offerings mentioned in ויקרא), however, is as the guilt-offering, therefore if he takes the fistful out of it not doing it as such, it is still valid (Sifra, Tzav, Chapter 3 3; Zevachim 11a).