Tractate Tahorot - Chapter 4 - Mishnah 3 טהרות פרק ד׳ משנה ג׳

A weasel that had in its mouth a [dead] sheretz or a dog that had carrion in its mouth and they passed between clean [persons] or if clean persons passed between them, their condition of doubt is deemed clean, since the uncleanness , had no resting place. If they were picking at them while these lay on the ground, and a person stated, "I went to that place but I do not know whether I did or did not touch it," his condition of doubt is deemed unclean, since the uncleanness had a resting place.