To cite CausalImpact in publications use:

Brodersen et al., 2015, Annals of Applied Statistics. Inferring causal impact using Bayesian structural time-series models. http://research.google.com/pubs/pub41854.html

Corresponding BibTeX entry:

  @Article{,
    title = {Inferring causal impact using {B}ayesian structural
      time-series models},
    author = {Kay H. Brodersen and Fabian Gallusser and Jim Koehler and
      Nicolas Remy and Steven L. Scott},
    journal = {Annals of Applied Statistics},
    year = {2014},
    volume = {9},
    pages = {247--274},
    url = {http://research.google.com/pubs/pub41854.html},
  }