To cite sampleSelection in publications use:

  Ott Toomet, Arne Henningsen (2008). Sample Selection Models in R:
  Package sampleSelection. Journal of Statistical Software 27(7). URL
  https://www.jstatsoft.org/v27/i07/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Sample Selection Models in {R}: Package {sampleSelection}},
    author = {Ott Toomet and Arne Henningsen},
    journal = {Journal of Statistical Software},
    year = {2008},
    volume = {27},
    number = {7},
    url = {https://www.jstatsoft.org/v27/i07/},
  }