\documentclass{article}
\usepackage[pdftex]{graphicx}
\usepackage[paper=letterpaper, text={4in,5in},centering]{geometry}

\makeatletter
   \renewcommand{\@biblabel}[1]{}
   \newcommand{\citenum}[1]{\def\@cite##1##2{##1}\cite{#1}}
\makeatother

\begin{document}

References \citenum{Knuth-1984}, \citenum{Lamport-1994}, and \citenum{Gratzer-2007} are useful for learning {\TeX} and \LaTeX.

\bibliography{example10}
\bibliographystyle{example10}

\end{document}

