Hillel on the Golden Rule. The Babylonian Talmud, Tractate Shabbath, Folio 31a. Translated and Edited by Rabbi Dr. Isidore Epstein. London: Soncino Press, 1935-1948. Also here and here.
another occasion it happened that a certain heathen came before Shammai and
said to him, “Make me a proselyte, on condition that you teach me the whole
Torah while I stand on one foot.” Thereupon he repulsed him with the builder’s
cubit which was in his hand. When he went before Hillel, he said to him, “What
is hateful to you, do not to your neighbour: that is the whole Torah, while the
rest is the commentary thereof; go and learn it.”