How do I display serial number in reference section in Beamer

Question

This is my preamble

\documentclass[9pt]{beamer}
   \usepackage[utf8]{inputenc}
   \usepackage[T1]{fontenc}
   \usepackage{lmodern}
   \usetheme{Goettingen}
   \usecolortheme{whale}
   \usepackage{verbatim}

And this is the bibliography

    \begin{frame}{\LARGE References}
    \begin{thebibliography}{9}
        \bibitem{S1}
        \small{Source 1}
    \bibitem{S2}
    \small{Source 2}

    \bibitem{S3}
    \small{Source 3}

    \bibitem{S4}
    \small{Source 4}
    \bibitem{S5}
    \small{Source 5}
    \bibitem{S6}
\small{Source 6}    

\end{thebibliography}
\end{frame}

In the slide it displays like this

However I couldn't figure out how to display numbers instead of this paper symbol. Any help is appreciated.