Season 2, Episode 12, "The Royale." This was subsequently tangentially referenced in an episode of "Deep Space Nine," which means "explanations" about Wiles never being born in the timelines are wrong.

Quoting from Wikipedia:

    In "The Royale", an episode (first aired 27 March 1989)
    of Star Trek: The Next Generation, Captain Picard states
    that the theorem had gone unsolved for 800 years. At the
    end of the episode Captain Picard says,

        "Like Fermat's theorem, it is a puzzle we
            may never solve."

    Wiles' proof was released five years after the particular
    episode aired. This was subsequently mentioned in a Star
    Trek: Deep Space Nine episode called "Facets" during June
    1995 in which Jadzia Dax comments that one of her previous
    hosts, Tobin Dax, had

        "the most original approach to the proof
            since Wiles over 300 years ago."

    This reference was generally understood by fans to be a
    retroactive continuity for "The Royale".