Is it grammatically correct to say,
"What a silly question of you..."
My friend is trying to convince me that
"What a silly question by you..."
is the only correct way.
"By you," is unnecessary; the preposition and subject are implied by the exclamatory nature of the sentence, i.e.: What a silly question!
In this instance, the words "by" and "of" are not synonymous. If I say, "I have a question of you," I am asking you to answer a question. If I say, "I have a question by you," the implication is that I have been given a question from you.
I would use neither in the case of the example provided.
'What a silly question by you' sounds awkward because it sort of emphasizes the fact that the question came from the questioner. As for 'what a silly question of you', this feels like an incomplete phrase to me. I would say 'what a silly question of you to ask'. That way, the sentence contains a verb and more focus is directed toward the asking of the question than the person it came from.
"What a silly question from you" works okay, in the same way. "What a silly question" turns the question very much into an object, in this case, characterised by silliness. Entire, as such. It's important to use an appropriate preposition. By and of don't free it up enough for that (I feel).
I also get the feeling here that the of-sentence implies "you" is being asked, and in the by-question, "you" is doing the asking. Very not-clear!
"of yours" would be best here. Sometimes if a sentence sounds strange no matter how you attempt to write it, simply reword it entirely.
"You asked such a silly question!" "What a silly question!" (of yours might be implied within the dialogue's context) "Well, that's a silly question, John!" (addressing the questioner by name).
...or any number of other possible sentences expressing this idea.
"of you" would be used when what is being spoken about consists of one item "by you" would be used in response to more than one or numerous topics
So if there is only one question being asked, then the "of you" use would be appropriate.
IF there is more than one question incorporated into the scenario, then you would use "by you"
Example sentences: What a silly question of you. What interesting questions by you.