Calvin and Hobbes has this done frequently by at least Calvin and his dad, and by Hobbes, who provides the page image.
Also pretty common in Peanuts, except that there a character would raise his or her entire fist. (Apparently, Charles Schulz didn't realize that this is a common socialist gesture.)
A folk tale has a peasant be mistaken for a doctor through a series of misunderstandings. When several scholars visit to test the renowned intelligence of this doctor, he lies in bed pretending to have lost his voice. One of the scholars goes in and holds his finger up, ready to launch into a long speech on how there is only one God. The peasant thinks it stands for "I'll poke your eye out", and responds with holding three fingers out for "I'll poke out both your eyes and cut off your nose". The scholar returns astonished, for as he was about to prove there was only one God, the doctor cut it short with the Father, the Son and the Holy Spirit.
"Weird Al" Yankovic does this in the "Dare To Be Stupid" video. Of course, this being Weird Al, he was emerging from a large pot with mashed potatoes in his hair, expression completely deadpan, while proclaiming "Mashed potatoes can be your friends."
Zuko: How am I supposed to convince these people that I'm on their side? What would Uncle do? [Impersonating his Uncle, pacing and holding up his finger] Zuko, you must look within yourself to save yourself from your other self. Only then will your true self reveal itself. [Dropping the impersonation and getting frustrated] Even when I'm talking for him I can't figure out what he means!
The Smurfs: Brainy Smurf does it every time he lectures the other Smurfs.
Practical Pig in Disney's Three Little Pigs. The finger point, in fact, was very common in the early days of The Golden Age of Animation, as mentioned on The Illusion of Life. Animators were starting to animate dialogue, and the pointing finger was the most popular way of hitting the accents on the voice track. As their skills improved and found more sophisticated ways of matching the vocals, use of the finger was curtailed as too cliche.
This probably goes back to Plato (the quintessential Western philosopher), as depicted on the famous fresco The School of Athens by Raphael. In fact, Plato's upward-pointing finger is contrasted by Aristotle's downturned palm, which symbolically depicts Plato's idealized view of nature and Aristotle's more materialistic way of thinking.