labour of love

/L EY1 B AW0 R AH1 V L AH1 V/
noun
  1. 1

    A task performed voluntarily without expectation of reimbursement; an altruistic work or undertaking.

    โ€œWriting a textbook has become more a labor of love than a money-making enterprise, wrote the professor, because of the peculiar way the books are now marketed.โ€

Source: https://en.wiktionary.org/wiki/labor%20of%20love, https://en.wiktionary.org/wiki/labour%20of%20love