Someone told me that you shouldn’t ask a question to someone who won't know the answer to what you're asking. What’s the source of this?
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אֲמַר לֵיהּ רַבִּי חִיָּיא לְרַב: בַּר פַּחֲתֵי! לָא אָמֵינָא לָךְ, כִּי קָאֵי רַבִּי בְּהָא מַסֶּכְתָּא לָא תְּשַׁיְּילֵיהּ בְּמַסֶּכְתָּא אַחֲרִיתִי, דִּילְמָא לָאו אַדַּעְתֵּיהּ. דְּאִי לָאו דְּרַבִּי גַּבְרָא רַבָּה הוּא — כַּסֵּפְתֵּיהּ, דִּמְשַׁנֵּי לָךְ שִׁינּוּיָא דְּלָאו שִׁינּוּיָא הוּא.
Rabbi Ḥiyya said to Rav, his sister’s son: Son of great men, didn’t I tell you that when Rabbi Yehuda HaNasi is involved in this tractate do not ask him questions in another tractate, as perhaps it will not be on his mind and he will be unable to answer? The dilemma that Rav asked was not related to the subject matter of the tractate which they were studying. As, had it not been for the fact that Rabbi Yehuda HaNasi is a great man, you would have shamed him, as he would have been forced to give you an answer that is not an appropriate answer.
See the Shulchan Arach Choshen Mishpat 228 which discusses onas devorim
אם נשאלה שאלה על דבר חכמה לא יאמר למי שאינו יודע אותה חכמה מה תשיב בדבר זה וכן כל כיוצא בדברים אלו
If a question was asked on a topic that requires wisdom you may not say someone who is unfamiliar with that subject "what would would you answer" on this matter.
This is also written in the Rambam
The Be'er Hagola says that although there is no clear source for this in the Gemora it is self understood based on the other examples of onas devorim given there