Sourdough of terumah and of kilei hakerem that fell into a dough, [and] there was not in this one sufficient to leaven and there was not in this one sufficient to leaven, [but] they combined and leavened, it is prohibited to non-Kohanim, and permitted to Kohanim. R' Shimon permits [it] to non-Kohanim and to Kohanim.