Kiddushin 65b: If one witness told someone, “Your food became tamei,” and the owner of the food disputes this, the food is tahor.
קידושין סה ע”ב: ואמר אביי: אמר לו עד אחד נטמאו טהרותיך, והלה שותק ־ נאמן, ותנא תונא: עד אחד אומר נטמאו, והלה אומר לא נטמאו ־ פטורֹ טעמא דאמר לא, הא אישתיק מהימן.
יו”ד קפ”ה ס”ג רמ”א: אמרה פלוני חכם טהר לי כתם והחכם אומר שהיא משקרת החכם נאמן וטמאה היא.
There was once a restaurant owner whose mashgiach told him that he found treif food in the restaurant. The man went to his rav, who said, “You don’t have to believe the mashgiach, because he is one witness coming to forbid something.”
The mashgiach called his rav hamachshir, Rabbi Yisroel Reisman, who asked his rebbe, Rav Avrohom Pam. Rav Pam said immediately: “That’s the Teshuvos Meishiv Davar Siman Alef.”
The Meishiv Davar is based on a Rema in the laws of Niddah, YD 185:3. If a woman tells her husband that a certain rabbi looked at her stain and permitted it, and then the husband goes and asks that rabbi if he really said that, and the rabbi says no, then she’s forbidden to him. Now, why is the husband obligated to listen to him? He is one witness coming to forbid her! Let him listen to his own wife, who is one witness to permit herself! The answer is that she is not claiming to know independently that she is permitted; she made her credibility dependent on this rabbi. Here too, the restaurant owner hired this mashgiach and based his credibility on him. He is not allowed to deny the mashgiach’s claim.
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