Rabban Shimon ben Gamliel says: [If] one buys a nursing animal from a non-Jew, he need not suspect that it was the offspring of another animal. [If] he entered among his flock and saw that those that had just borne for the first time were nursing, and that those that had previously given birth were nursing [as well], he need not suspect that the offspring of this one came to that one, or that the offspring of that one came to this one.