Mishnah 2 - Chapter 4 - Pirkei Avot

Ben Azzai says: Run to a "light" mitzvah as to a "heavy one," and flee sin. For a mitzvah begets a mitzvah and a sin begets a sin. [This is "the way of the world." If a man does one mitzvah, it is easier for him to do others and if one begins sinning, it is difficult for him to stop. And, what is more,] the reward for a mitzvah is a mitzvah, and the "reward" for a sin is a sin. [For Heaven assists, and places in the hand of one who has done a mitzvah the wherewithal to do another, in order to reward him for both; and thus with the "reward" for a sin. Or: All that one earns or enjoys in the doing of a mitzvah is reckoned to him as a mitzvah itself, and he receives reward both for the mitzvah and for the pleasure and the benefit that he derived therefrom; and the "reward" and the enjoyment that one derives from sinning is reckoned to him as a sin itself, and he is smitten both for the sin and for the reward and the pleasure that he derived from it.]