Rashi on Leviticus 7:1/רש"י על ויקרא ז׳:א׳

1קדש קדשים הוא [THIS THE LAW OF THE GUILT OFFERING] IT IS MOST HOLY — “It is most holy”; it may be offered, but an animal that is exchanged for it (cf. Leviticus 27:34) may not be offered (Sifra, Tzav, Section 5 2).