To cite rtweet use:

Kearney, M. W. (2018). rtweet: Collecting Twitter Data. R package version 0.6.7 Retrieved from https://cran.r-project.org/package=rtweet

Corresponding BibTeX entry:

  @Manual{rtweet-package,
    title = {rtweet: Collecting Twitter Data},
    author = {Michael W. Kearney},
    year = {2018},
    note = {R package version 0.6.7},
    url = {https://cran.r-project.org/package=rtweet},
  }