Manava Dharmashastra 2.3

2.3

saṃkalpamūlaḥ kāmo vai yajñāḥ saṃkalpasaṃbhavāḥ
vratāni yamadharmāś ca sarve saṃkalpajāḥ smṛtāḥ //

The desire (for rewards), indeed, has its root in the conception that an act can yield them, and in consequence of (that) conception sacrifices are performed; vows and the laws prescribing restraints are all stated to be kept through the idea that they will bear fruit.