Mbh 3.31.1c

O daughter of Drupada, religion is the only raft for those desirous of going to heaven, like a ship to merchants desirous of crossing the ocean. O thou faultless one, if the virtues that are practised by the virtuous had no fruits, this universe then would be enveloped in infamous darkness. No one then would pursue salvation, no one would seek to acquire knowledge not even wealth, but men would live like beasts. If asceticism, the austerities of celibate life, sacrifices, study of the Vedas, charity, honesty,--these all were fruitless, men would not have practised virtue generation after generation. If acts were all fruitless, a dire confusion would ensue. For what then do Rishis and gods and Gandharvas and Rakshasas who are all independent of human conditions, cherish virtue with such affection? Knowing it for certain that God is the giver of fruits in respect of virtue, they practise virtue in this world. This, O Krishna, is the eternal (source of) prosperity. When the fruits of both knowledge and asceticism are seen, virtue and vice cannot be fruitless. Call to thy mind, O Krishna, the circumstances of thy own birth as thou that heard of them, and recall also the manner in which Dhrishtadyumna of great prowess was born! These, O thou of sweet smiles, are the best proofs (of the fruits of virtue)! They that have their minds under control, reap the fruits of their acts and are content with little. Ignorant fools are not content with even that much they get (here), because they have no happiness born of virtue to acquire to in the world hereafter. The fruitlessness of virtuous acts ordained in the Vedas, as also of all transgressions, the origin and destruction of acts are, O beautiful one, mysterious even to the gods. These are not known to any body and everybody. Ordinary men are ignorant in respect of these. The gods keep up the mystery, for the illusion covering the conduct of the gods is unintelligible.