That particular line was indeed a Looney Tunes character, (Merrie Melodies actually) you are probably thinking of "Of Fox and Hounds", a Tex Avery cartoon loosely based on John Steinbeck's novel "Of Mice and Men". Avery lifts the peronalities of the books' characters, George, a small, smart man, and Lennie, a large, mentally challenged man, and puts their personalities into a fox and a dog (In this case, his name is "Willoughby", though). Anyway, "George" tricks Willoughby again and again, by dressing as another dog, and when Willoughby asks "Which way did he go, George"? George tells him "that way", and gives him directions to jump over a fence, leading Willoughby off a cliff. After Willoughby falls for it multiple times, he does it again at the end of the cartoon, only this time we don't hear a crash....cut to the end of the film, with the dog lying on a pile of mattresses..."I aint so dumb!"