Tosefta zu Bamidbar 18:7

Said Rabbi Nechemiah, why is everyone trusted as to [the purity of the articles involved in] the purification rite [involving the red heifer] and as to consecrated food ("נאמנין על בהחטאת ועל הקדש", see Lieberman), but not everyone is trusted as to Terumah? So that people will not say, "Behold, I am building an altar for myself," or "Behold, I am burning a red heifer for myself," as it is said (Num. 18:7), "And you (=Aaron) and your sons with you shall safeguard your priesthood in every matter [regarding] the Altar..." (See Chag. 22a:15.) One might say that [the word "safeguard" includes] even [Terumah,] Terumat Ma'aser (the priestly portion taken from a Levite's tithes) and Challah (the portion of dough set aside for the Kohen), [and thus] the Torah comes to teach "... and to [that which is] within the Curtain." What is special about that which is "within the Curtain" is that it is not familiar to [the non-priestly class of] Israelites [who are forbidden from entering the area beyond the Curtain], which thus excludes Terumah and Terumat Ma'aser and Challah, with which Israelites are familiar. Rabbi Yehuda says, behold, it says [in regards to the red heifer] (Num. 19:9), "and it shall be for the congregation of the Children of Israel to safeguard it as the waters of purification...", such that everyone should be trusted as to safeguarding it. [If there is] a doubt as to the status of an am ha'aretz as regards to [the impurity of] treading or movement [of a zav (see Niddah 43a:3)], [they are deemed] pure as to matters of chullin and impure as to matters of Terumah.