I heard this brief tale at a dinner as part of the before-meal blessing. It seems this particular Rabbi also got knocked from his trajectory as he went about his day:

One ordinary day a few millenia ago a rabbi walked to a nearby village to purchase supplies. On his way home, he absentmindedly took the wrong path. Suddenly a voice challenged him from the darkness: “Who are you, and why are you here?” Realizing he had wandered into a nearby garrison and that the voice had come from the young sentry standing post, the startled Rabbi (setting the precedent for all Rabbis thereafter) answered the question with another question: “Sir, how much do they pay you to stand guard and ask that question to all who approach?” The sentry, seeing that this was a rabbi and not an intruder, responded meekly “Five drachmas a week, sir.” The rabbi replied, “Young man, I will DOUBLE your pay if you come with me, stand in front of my cottage, and ask me that question each morning as I begin my day: Who are you, and why are you here?”