@Article{john_doe,
 author = {{Doe}, J.},
 title = "{it John's Doe book}",
 journal = {Book or journal},
 year = {2014},
 volume = "{bf 1}",
 pages = {5--10},
}

@Article{foo_bar,
 author = {{Bar}, F.},
 title = "{it Bar, bar book}",
 journal = {Book or journal},
 year = {2014},
 volume = "{bf 1}",
 pages = {5--10},
}

documentclass[12pt]{report}
usepackage{natbib}

begin{document}

Some text here citep[cited by][]{john_doe}, and some other text by cite{foo_bar}.

bibliographystyle{unsrt}
bibliography{ex}{}

end{document}

documentclass[12pt]{report}
usepackage{natbib}

begin{document}

Some text here citep[cited by][]{foo_bar}, and some other text by cite{john_doe}.

bibliographystyle{unsrt}
bibliography{ex}{}

end{document}