Reporting Elo Boosting

So I'd like to know how I can report someone I suspect for elo boosting. I got dominated by a riven player (wait for it), and his mechanical gameplay just seemed too good for my elo. I looked on his account, He's b5. Not only that, but his match history is like, 7 losses with random champs, then all of a sudden hard carry wins with Riven, out of nowhere. Like that doesnt happen over night. So yea, how do make investigate?

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