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.

Yoezer, a man of the Birah, was of the students of Beis Shammai, and he said: I asked Rabban Gamliel the Elder while standing at the East Gate, and he said, “It never forbids unless there is enough of it to leaven.”