Beitzah 28A

מתני׳ רבי יהודה אומר: שוקל אדם בשר כנגד הכלי או כנגד הקופיץ. וחכמים אומרים: אין משגיחין בכף מאזנים כל עיקר.

Mishnah: R' Yehuda says: A person selling meat on a Festival who wishes to know its weight in order to determine its price may not weigh it against regular weights in the ordinary weekday manner, but he may weigh the meat against a vessel or against a cleaver [kofitz] and then calculate the weight of the meat by weighing the vessel or cleaver later. And the Rabbis say: One may not look at the pans of a balance scale at all, meaning that they may not be used for weighing in any manner or for any other purpose.

גמ׳ מאי ״כל עיקר״? אמר רב יהודה אמר שמואל: אפילו לשמרו מן העכברים. אמר רב אידי בר אבין: והוא דתליא בתריטא.

Talmud: The Talmud asks: What is the meaning of: One may not look at the pans of a scale at all? Rav Yehuda said that Shmuel said: Even if one has meat and he wishes to protect it from mice, he may not put it on a scale, since it would look as if he were weighing it. Rav Idi bar Avin said: And this ruling applies only where the scale is hanging on the ring used for balancing it. However, if the scale is hanging in such a manner that it is not fit for weighing, one may use it as he would use any other vessel.

ואמר רב יהודה אמר שמואל: טבח אומן אסור לשקול בשר ביד. ואמר רב יהודה אמר שמואל: טבח אומן אסור לשקול בשר במים.

And Rav Yehuda said that Shmuel said: It is prohibited for an expert butcher to weigh meat on a Festival even by hand because he can arrive at its precise weight using this method, and so it is prohibited like weighing with a scale. And likewise, Rav Yehuda said that Shmuel said: It is prohibited for an expert butcher to weigh meat on a Festival even in water. Butchers would calculate the weight of a slab of meat by placing it into a tub of water and measuring the amount of water that it displaces with the help of markings on the container. Since this procedure enables the butcher to determine the precise weight of the meat, it may not be performed on a Festival.