Sourdough of chullin and of terumah 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, R' Eliezer says: “We follow the last one.” But the Sages say: Whether the prohibition fell in the beginning or at the end, it never forbids until in it there is sufficient to leaven.