The Baraisa Kinyan Torah (Pirkei Avos ch. 6) no. 3 states:
One who learns from his fellow a single chapter [of Torah], a single law, a single verse, a single statement, or even a single letter must treat him with honor. For so we find with David, King of Israel, who learned from Ahisofel two things alone and he called him his teacher, his guide, and his intimate, as it is said: "And you are a man of my worth, my guide and intimate" (Psalms 55:14). And does not this matter allow for an a fortiori (kal va-homer) logical deduction: If David, King of Israel, who learned from Ahisofel two things alone, called him his teacher, guide and intimate, one who learns from his fellow one chapter, one law, one verse, one statement, or even one letter, all the more so must he treat him with honor.This is a very difficult passage. King David learned two things from Ahisofel and respected him. Therefore, we deduce using a kal va-homer that one must respect someone who teaches him even one thing. However, since we are learning from two things, how can we deduce that it applies to even one thing? The deduction is flawed!
The Maharal offers two solutions to this difficulty. Both center on the idea that we are not deducing from David about over how much Torah one must show respect to a teacher. That is learned from elsewhere. What we see from King David is either that he did not really learn anything substantial from Ahisofel but still respected him, so kal va-homer one who learns actual Torah from another. Alternatively, we see that even a king -- who commands respect himself -- must respect someone who teaches him Torah, so kal va-homer that we commoners must respect a Torah teacher.