Mishnayos Avos Perek 4 Mishnah 2

"Since one commandment leads to another commandment": That is the way of the world, that one who does one commandment finds it easier to do others; and one who starts to do sins will find it difficult to separate from them. And also, "since the reward for a commandment is [another] commandment;" [meaning it] is from the Heavens that the one who does one commandment is helped and another commandment is placed in front of him to [also] do, in order to give him the reward for both of them. And so too, "the reward of a sin, etc." Another explanation: "since the reward for a commandment is [another] commandment;" Since all that a person is rewarded and derives enjoyment while doing a commandment is itself considered a commandment for him. And [so] he gets reward for the commandment that he did and for the enjoyment and benefit that he experienced in doing it.

"and the reward for one sin is another sin": And the reward and benefit that one derives from doing a sin is considered like a sin itself for him. And [so] he is [punished] for the sin that he committed and for the reward and benefit that he received by doing it.