To cite fitdistrplus in publications use:

  Marie Laure Delignette-Muller, Christophe Dutang (2015).
  fitdistrplus: An R Package for Fitting Distributions. Journal of
  Statistical Software, 64(4), 1-34. DOI 10.18637/jss.v064.i04.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{fitdistrplus}: An {R} Package for Fitting Distributions},
    author = {Marie Laure Delignette-Muller and Christophe Dutang},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {64},
    number = {4},
    pages = {1--34},
    doi = {10.18637/jss.v064.i04},
  }

Please cite both the package and R when using them for data analysis.
See also ‘citation()’ for citing R.