Parah, Chapter 8, Mishnah 4. Throughout this chapter the Mishnah collects situations in which tumah moves along a surprising path, and every one of them closes with the same arresting line: the thing that gave you your tumah could not have given me mine, and yet from you I became tamei. Here that line is applied to a category of tumah unlike any other in the Torah: the carcass of a bird of a kosher species.
The Mishnah's words are: "Ha'ochel minivlas ha'of hatahor", one who eats from the carcass of a tahor bird, "vehi beveis habeli'ah", once the food has reached his gullet, "metamei begadim", he conveys tumah to clothing. And then the contrast: "haneveilah atzmah", the carcass on its own, "einah metam'ah begadim", has no power to make clothing tamei at all.
The picture is this. A person eats a kezayis, an olive's bulk, of the carcass of a kosher bird. The moment that piece reaches the beis habeli'ah, the gullet, he becomes tamei, and his tumah is strong enough to pass into the clothing he is wearing. The carcass itself, however, has no such power. Touching it or carrying it does nothing at all.
The source is the pasuk that speaks of one who eats neveilah, a creature that died on its own, or treifah, one torn by a beast: "Every soul that eats neveilah or treifah shall wash his garments and bathe in water, and he shall be tamei until evening, and then he shall be tahor." The chachamim understood this verse to be speaking of nivlas of tahor, the carcass of a kosher bird, which becomes forbidden as neveilah or as treifah.
Two halachos emerge from this verse. The first is that this particular tumah is triggered by eating alone; no amount of handling or carrying the carcass will bring it on. The second concerns the exact moment of onset. Scripture writes "kol nefesh asher tochal", every nefesh that eats, and Chazal read the word nefesh as a hint to where in the body the process must reach: the beis hanefesh, the region in which a person's nefesh derives its enjoyment from what he eats, meaning the gullet. Before the food arrives there, nothing has taken effect; from the instant it arrives, the eater is tamei and the clothes on him are drawn into that tumah with him.
The full treatment of the halachos of a tahor bird that became neveilah is found in Maseches Taharos, in the opening mishnah of that tractate.
The Mishnah therefore signs off with the refrain that runs through this chapter: "Harei zeh omer", this one declares, "metamecha lo tim'uni", whatever it was that made you tamei had no ability to do so to me, "va'atah timtani", and still it is from you that I have become tamei. The clothing is speaking to the man who ate: had that bird's carcass come into contact with us by itself, we would have remained perfectly tahor; only after it passed through your eating did it acquire the strength to reach us.