"To protect the sheep you gotta catch the wolf, and it takes a wolf to catch a wolf" is a line from which movie?

In the 2001 film, Training Day, Alonzo Harris says to Jake Hoyt, "To protect the sheep you gotta catch the wolf, and it takes a wolf to catch a wolf." Denzel Washington is the veteran detective, Alonzo Harris, a self-proclaimed "wolf among wolves." He is eager to teach his rookie partner Jake (Ethan Hawke) fixed rules (Alonzo's rules) that basically apply to urban policing.

If the rules aren't Alonzo's rules, then they don't apply on the mean streets of Los Angeles. Caught in a web of deception, Jake watches with escalating horror as Alonzo uses his badge with the support of his superiors to apply self-righteous justice and universal corruption.

Now in stark contrast to work in most of his previous films, Denzel unleashes his dark human side as Alonzo Harris. With fearlessness and fury, Denzel's acting according to top critics results in excellence without compromise. Also the critics say that Director Antoine Fuqua (The Replacement Killers, 1998) doesn't get any points for subtlety. Fuqua supplied gritty details (using actual L.A. gang members as extras). Plus, Hawke's finely tuned performance which the director procured was perfectly matched to Washington's frightening volatility.

Overall the powerhouse performance by Washington fuels this brutal urban police film, in which a rookie narcotics cop learns the hard way that even very good cops can go very, very bad. Cops are not "mean ferocious" wolves. Each good cops has a moral compass and meets fiduciary duties.