Walter Hooper tells this one in his book, C. S. Lewis: A Companion & Guide. Keep in mind, Lewis is only nine in this story.
It was also a young boy that a future C. S. Lewis, champion of Reason,
emerged. Warren Lewis [his brother] has given us an example of that
'dexterity of riposte' for which his brother later became so famous. It
was 1907 and the family was preparing for a holiday in France. Warren
said, 'Entering the study, where my father was poring over his account books,
Jack flung himself into a chair and observed, "I have a prejudice against the
French." My father, interrupted in a long addition sum, said irritably,
"Why?" Jack, crossing his legs and putting his finger tips together,
replied, "If I knew why it would not be a prejudice"' (Hooper Companion & Guide 4)
Again, Walter Hooper tells this one in the introduction of The Weight of Glory. This is one of the stories toward the end of Lewis's life after he had awaken from his coma.
Even before he went into the nursing home I marvelled that Lewis had lived
so long without setting himself ablaze. Except when he dressed for a
special occasion, he wore an old tweed jacket, the right-hand pocket of
which had been patched and re-patched many times. This was because
Lewis, when wearied of his pipe, would drop it into his pocket, with the
results that it would burn its way through. And this happened so often
that there was none of the original material left.
The nurses in the Acland, having found him nodding wtih a cigarette in
his hand, would have none of this. And so it was that, except when I
was with him, they would not allow him to have any matches. What puzzled
Lewis was that after I had left him with a box of matches, a nurse, would,
as soon as I left, rush in and take them away. "How do they know?" he asked me one morning. "Give me a box I can hide under my bedclothes." I had then to confess that while I was the supplier, I was also the informer. "Imformer!" roared Lewis. "I have what no friend ever had before. I have a private traitor, my very own personal Benedict Arnold. Repent before it
is too late!" (Lewis Weight of Glory 5-6)
As much as I like to read stories by Lewis, I love hearing stories about him. Last year, I was able to sit and listen to Walter Hooper tell many stories (these two included) about Lewis. I don't think I will ever forget it. Here's to you, C. S. "Jack" Lewis: November 29, 1898-November 22, 1963.