Is there any source to the idea that one should not make some bracha with a knife in one's hand?

Question

Is this situation arises on Shabbos by Hamotzi over a challah?

Answer 1

Yes, something similar is ruled regarding Birkat Hamazon. See Bet Yosef Orach Chayim siman 180, paragraph 5. here is some quotes of Bet yosef and Shulchan aruch and Acharonim.

כתב הרוקח בסימן של"ב מכסין סכין בשעת ברכת המזון על שם "לא תניף עליהם ברזל" (דברים 27, 5). במכילתא (פרשה י"א הלכה ח') אינו דין שיניף וכו' המקצר המקצר על המאריך ושלחן כמזבח בשילהי חגיגה (כז עמוד א') עד כאן לשונו וכן כתב הרב דוד אבודרהם וכן כתוב בשבלי הלקט (סימן קנ"ה) וכתב עוד בשבלי הלקט מפי החבר רבי שמחה שמעתי טעם אחר פעם אחת היה אחד מברך ברכת המזון וכשהגיע לברכת בונה ירושלים ונזכר חורבן הבית לקח הסכין ותקעו בבטנו ועל כן נהגו לסלקו בשעת ברכה. ושני טעמים אלו כתובים בארחות חיים (הלכות ברכת המזון סעיף ח') ונוהגין בשבתות וימים טובים שלא לכסות הסכין ולפי טעם רבי שמחה אין לחלק בין שבת לחול ומכל מקום מנהגן של ישראל תורה היא. ‏

A summary in English: The Rokeach wrote that they cover the knife at Birkat Hamazon time, except for Shabbat and Yom Tov. Two explanations are given. The first, concerning the similarity between the table and Mizbeach (the altar of Mishkan and Mikdash) which do not touch iron (at time of its construction), because the first will lengthen life and the second shorten it. The second explanation is that one time, when blessing Bone Yerushalayim, a man remembered the Temple destruction, take the knife and stabb itself. Despite that the second explanation doe's not make difference between Shabbat and Yom Tov and the last days, the custom is to cover the knife on week days only. He summarize this rule for week days in Shulchan Aruch.

The Magen Avraham sk 4 explained that the first explanation concerns the construction of Mizbeach, and at Shabbat and Yom Tov we are prohibited to build anything, so there is not allusion to mizbeach. He wrote also that the Levush had as cultom to cover the knife on S. and YT too.

But mishna Berura sk 12 rules as Shulchan Aruch.

Conclusion:

A similar custom exists, to cover the knife at time of Blessing after the meal (Birkat Hamazon), and this custom seems not to be widespread at Shabbat and Yom Tov. I don't know if some people nowaday has the custom reported in Levush (that seems linked to Shibole Haleket) to cover the knife before birkat hamazon even on Shabbat and Yom Tov. Covering Hamotsi is not mentionned in the texts above cited (following the second explanation it is obvious, second the first it is obvious for Shabbat and Hollidays only but anyway nobody of the above cited poskim wrote about Hamotsi)

I just see that @user6591 understand too that this minhag is the source of the question.