I have heard since childhood that one always has to pay for his paap or punya karma even when no one is watching. Therefore one should be good to every creature even though it cannot harm us. My mother used to say that if we kill an animal, in some next birth we will be that animal and be killed in the same manner. I think this is more generally know as law of karma. Please correct me if I'm wrong.
In which Hindu scripture can I find this written that one always has to pay for his karma? Does there exist a Vedic source (like major Upanishads, Brahman or Samhita)?