משנה על תרומות 5:1

Thirteen rulings govern the carrion of a clean bird:There must be intention; It need not be rendered susceptible; It conveys food uncleanness if its minimum bulk is that of an egg; And it conveys uncleanness when in one's gullet if its minimum bulk is that of an olive; He that eats of it must wait until sunset [to be clean]; Guilt is incurred on account of it for entering the sanctuary; Terumah is burned on account of it; He who eats a limb of it while it is alive suffers forty lashes; Slaughtering it or nipping [off its neck] cleanses it even if it is terefah, the words of Rabbi Meir. Rabbi Judah says: they do not cleanse it. Rabbi Yose says: the slaughtering does cleanse it but nipping does not.

Concerning what did they say: “Anything that causes fermentation or seasons or which renders medumma we rule stringently”? [In the case of] a species [mixed] with its [like] species. [When did they say] “we rule [sometimes] leniently and [sometimes] stringently”? [In the case of] a species [mixed] with a different kind of species. How so? If leaven of wheat fell into dough of wheat and there is enough to cause fermentation, [then] whether there is enough to become neutralized in one-hundred-and-one, or there is not enough to become neutralized in one-hundred-and-one, it is prohibited. If there is not enough to become neutralized in one-hundred-and-one, [then] whether there is enough to cause fermentation, or there is not enough to cause fermentation, it is prohibited.