To cite fastcluster in publications use:

Daniel Müllner (2013). fastcluster: Fast Hierarchical, Agglomerative Clustering Routines for R and Python. Journal of Statistical Software, 53(9), 1-18. URL http://www.jstatsoft.org/v53/i09/.

Corresponding BibTeX entry:

  @Article{,
    title = {{fastcluster}: Fast Hierarchical, Agglomerative Clustering
      Routines for {R} and {Python}},
    author = {Daniel M\"ullner},
    journal = {Journal of Statistical Software},
    year = {2013},
    volume = {53},
    number = {9},
    pages = {1--18},
    url = {http://www.jstatsoft.org/v53/i09/},
  }