Citing QGIS in formal publications?

Start here: https://qgis.org/en/site/getinvolved/faq/index.html#how-to-cite-qgis

Q: How to cite QGIS?

A: To cite QGIS software in your piece of work, for work or an assignment, this general citation might be helpful: "QGIS Development Team (YEAR). QGIS Geographic Information System. Open Source Geospatial Foundation Project. http://qgis.osgeo.org".

from https://hub.qgis.org/wiki/quantum-gis/QGIS_Citation_Repository

bibtex:

  @Manual{QGIS_software,
    title = {QGIS Geographic Information System},
    author = {{QGIS Development Team}},
    organization = {Open Source Geospatial Foundation},
    year = {2009},
    url = {http://qgis.org},
  }

which gives

QGIS Development Team, 2009. QGIS Geographic Information System. Open Source Geospatial Foundation. URL http://qgis.org

EDIT: I updated the url