Korihor asked for a sign. What sign did Alma (and God) give him?

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He was struck dumb.

Korihor said unto him: ... ye do not know that there is a God; and except ye show me a sign, I will not believe.

Now Alma said unto him: This will I give unto thee for a sign, that thou shalt be struck dumb. ...

Now when Alma had said these words, Korihor was struck dumb. Alma 30:48-50