For the source text click/tap here: Zevachim 105
To download, click/tap here: PDF
The Gemara on our daf discusses the source for this halakha and quotes a baraita that explains that regarding the sacrifices brought on Yom Kippur the Torah sounds as if they must be removed and burned outside of a single encampment (see Lev 16:27), while regarding other such sacrifices the requirement is to burn them outside of three camps (the inner camp of the Tabernacle, the middle camp of the tribe of Levi and the outer camp of Israelites) in the desert.
From this we understand that although the sacrifices must be burned on the beit ha-deshen outside of all three camps (and in Temple times, outside the walls of the city of Jerusalem), nevertheless, the ritual defilement of the clothing of the kohanim associated with this service takes effect immediately upon leaving the encampment of the Tabernacle (or the Temple courtyard).