Тосефта к Бамидбар 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, not נאמנין <לא> על הקדש ולא על התרומה ("trusted neither as to consecrated foods nor as to Terumah") (see Lieberman), but not everyone is trusted as to Terumah? So that people will not say, "I will build an altar for myself," or "I will burn 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 and Challah^*the priestly portion taken from a Levite's tithes and the portion of dough set aside for the Kohen, respectively, [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^*i.e., one who is not generally trusted to adhere to laws of purity or tithing as regards to [the impurity of] treading or movement [of a zav^*i.e., one who has experienced an abnormal discharge], [they are deemed] pure as to matters of chullin and impure as to matters of Terumah.