The 107th prohibition is that we are forbidden from changing an animal designated as one category of sacrifice into another category — such as changing a peace-offering into a burnt-offering or a guilt-offering into a sin-offering. To do this or anything like it is a prohibition.
The source of this prohibition is G‑d's statement1 (exalted be He) regarding a firstborn animal, "A person may not sanctify it." The Oral Tradition explains:2 "The verse 'A person may not sanctify it' — as [another type of] a sacrifice."
The Sifra says: "[From this verse] I only know that a firstborn animal may not be changed from one level of sanctity to another. What is the source that this law applies to other sanctified animals? From the phrase 'from the animals — a person may not consecrate it.' "
This statement alludes to the verse,3 "A firstling animal that must be sacrificed as a firstborn offering to G‑d from the animals — a person may not sanctify it." [The superfluous phrase "from the animals" makes it] as if the verse says, "any type of animal which is an offering to G‑d may not be sanctified with another type of sanctity. Rather, it must be left as is."
The details of this mitzvah are explained in the 5th chapter of tractate Temurah.
