Midrash for Chullin 141:36

אשכחן טומאה בלועה טהרה בלועה מנלן

- Even so, granted that it is not fit for a stranger if it was swallowed in his presence, it is, however, fit for a stranger if swallowed not in his presence.<span class="x" onmousemove="('comment',' So where a person swallowed whole a morsel of nebelah without chewing it a moment before sunset and yet he is declared clean immediately after sunset, although the morsel if ejected again is fit to be eaten by a stranger who has not seen it in the mouth of another, the reason can only be that it is swallowed uncleanness and so cannot render unclean.');">²⁴</span>

Sifrei Devarim