Pirkei Avot 1:3

"Antignos of Socho received [the transmission] from Shimon the Righteous. He used to say: Do not be as servants who serve the Master to receive reward. Rather, be as servants who serve the Master not to receive reward. And let the fear of heaven be upon you."

The first sentence of Antigonus' teaching is easy to comprehend: We must not follow God's commands out of expectation of reward.  As we know, and as Rabbi Kushner has reminded us, good things don't always come to people who do good things.  In the words of the popular American saying, "goods deeds rarely (or never) go unpunished."  So why do good deeds?  The next sentence gives a murky answer: to be as servants who serve the Master not to receive reward.  Why would a servant serve a master with no expectation of reward?  There are two possibilities: fear of punishment for not serving, or love of the Master.  The final sentence suggests that it is both.  The key Hebrew word in the sentence, "Morah," is related to "Yara," fear, and to "Moreh," teacher.  It is not fear of a specific punishment for a specific transgression, because the text doesn't use the word "punishment," and the idea that all bad deeds are punished is as unsophisticated as the idea that all good deeds are rewarded.  Rather, it is the fear that negative consequences will come from a life of not aligning oneself with the dominant force in the universe, and at the same time the moral teacher of the universe.  We should not expect reward for serving God by following his commandments, but we should live with the belief that they come from the master and moral teacher of the universe.