A foreigner came to Shammai once, and said to him: "I will become a Jew if you teach me the entire Torah while I'm standing here on one foot." Shammai was furious, and chased him away with the builder's rule in his hand.

The man then went to Hillel, and told him the same thing. Hillel immediately made him a Jew by telling him: "What is hateful to you, do not do to another. That is the entire Torah. All the rest is commentary. Now go, and learn it."

(Talmud, Shabbat 31a)