\begin{filecontents*}{\jobname.bib}
@manual{sage,
key = {{\SAGE}},
organization = {The Sage Group},
title = {{Sage}: {O}pen Source Mathematical Software (version 5.10)},
note = {\url{sagemath.org}},
year = {2013},
}
\end{filecontents*}
\documentclass{amsart}
\usepackage{url}
\providecommand{\SAGE}{Sage}
\begin{document}
\cite{sage}
\bibliographystyle{amsalpha}
\bibliography{\jobname}
\end{document}