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\usepackage{amsmath,amssymb,amsthm}
\newtheorem{theorem}{Theorem}

\begin{document}

\begin{theorem}\label{thm:knuth}
...
\end{theorem}
Theorem~\ref{thm:knuth} is due to Donald~Knuth. For a reference, cf.~The~\TeX{}book.
\end{document}