To cite the R package 'cluster' in publications use:

Maechler, M., Rousseeuw, P., Struyf, A., Hubert, M., Hornik, K.(2017). cluster: Cluster Analysis Basics and Extensions. R package version 2.0.6.

Corresponding BibTeX entry:

    title = {cluster: Cluster Analysis Basics and Extensions},
    author = {Martin Maechler and Peter Rousseeuw and Anja Struyf and
      Mia Hubert and Kurt Hornik},
    year = {2017},
    note = {R package version 2.0.6 --- For new features, see the
      'Changelog' file (in the package source)},