“On 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.'”

It’s called “The Golden Rule” http://en.wikipedia.org/wiki/Ethic_of_reciprocity.