To cite StanHeaders in publications use:

  Stan Development Team (2018). StanHeaders: Headers for the {R}
  interface to {Stan}. https://mc-stan.org/.

A BibTeX entry for LaTeX users is

  @Misc{,
    title = {{StanHeaders}: Headers for the {R} interface to {Stan}},
    author = {{Stan Development Team}},
    note = {R package version 2.21.0-6},
    year = {2020},
    url = {https://mc-stan.org/},
  }