Tosefta do Terumot 10:2

Terumah sourdough that fell inside a batch of dough and was lifted up, and afterwards [made the dough] sour, it is permitted. Sourdough from seventh-year produce that fell inside a batch of dough, and he was aware of it, and afterwards it became sour, it is permitted (alt., "forbidden," see Raibag of Vilna). Terumah and seventh-year sourdough that [both] fell inside a batch of dough, and neither fell inside in order for it to cause leavening, and they got mixed up [in the dough], and it became leavened, it is forbidden to non-Kohanim. Rabbi Elazar son of Rabbi Shimon permits it to non-Kohanim. [If] this one fell inside in order to cause leavening, or that one fell inside in order to cause leavening, and they got mixed up, and it became leavened, it is forbidden even to Kohanim. Rabbi Elazar son of Rabbi Shimon permits it to Kohanim.