Beitzah 28B

מעבר לסכינא אפומא דדקולא, ואמרי ליה: לחדדה קא עביד מר, או להעביר שמנוניתה? ואמר לי: להעביר שמנוניתה. וחזיתי לדעתיה דלחדדה קא עביד, וקסבר: הלכה, ואין מורין כן.

running a knife over the rim of a basket on a Festival. And I said to him: Is the Master doing so in order to sharpen it or in order to remove its fat? And he said to me: To remove its fat. And I nevertheless perceived his intention, that he did so in order to sharpen it. He concealed this from me, however, because he holds that R' Yehuda’s opinion is the halakha but a public ruling is not issued to that effect; therefore he did not wish to permit the practice explicitly.

ואמר אביי: הוה קאימנא קמיה דמר, והוה קא מעבר סכינא אשפתא דרחיא. ואמרי ליה: לחדדה קא בעי מר, או להעביר שמנוניתה? ואמר לי: להעביר שמנוניתה. וחזיתי לדעתיה דלחדדה קא עביד, וקסבר: הלכה, ואין מורין כן.

Similarly, Abaye said: I was standing before my Master and teacher, Rabba, and he was running a knife over the edge of a millstone on a Festival. And I said to him: Does the Master wish to sharpen it, or is he doing this in order to remove its fat? And he said to me: To remove its fat. And I perceived his intention, that he did so in order to sharpen it, but he holds: This is the halakha, but a public ruling is not issued to that effect.

איבעיא להו: מהו להראות סכין לחכם ביום טוב? רב מרי בריה דרב ביזנא שרי, ורבנן אסרי. ורב יוסף אמר: תלמיד חכם רואה לעצמו, ומשאילה לאחרים.

A dilemma was raised before the rabbis: What is the halakha with regard to showing a slaughtering knife to a Sage on a Festival so that he may examine it for nicks and imperfections that would render it unfit to be used for slaughtering? Rav Mari, son of Rav Bizna, permitted it, whereas the Rabbis prohibited it, as this is somewhat similar to a weekday activity. And Rav Yosef said: A Torah scholar may examine a knife for himself in his own home and then lend it to others, who may also slaughter with it. Since the examination is not performed in public, it does not appear similar to a weekday activity.