Aragorn said it in "Flotsam and Jetsam". When Merry is telling the story of how Saruman's army marched out of Isengard, he concludes with:

"'...But there were some others that were horrible: man-high,
but with goblin-faces, sallow, leering, squint-eyed. Do
you know, they reminded me at once of that Southerner at
Bree; only he was not so obviously orc-like as most of
these were.'"

To which Aragorn replies:

"'I thought of him too,' said Aragorn. 'We had many of these
half-orcs to deal with at Helm's Deep. It seems plain now
that that Southerner was a spy of Saruman's; but whether he
was working with the Black Riders, or for Saruman alone, I
do not know. It is difficult with these evil folk to know
when they are in league, and when they are cheating one
another.'"

I rather thought the idiom would easily identify Aragorn, but I suppose Legolas does speak like him on occasion.

I didn't expect that finding the chapter would be quite so easy, but I was afraid to quote more of the passage for fear of making it TOO easy. :)

There is good reason why I don't make my living writing trivia quizes.

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The Fairy Tales of J.R.R. Tolkien