\cite[text]{key_list}
The key_list argument is a list of citation keys. This command
generates an in-text citation to the references associated with the keys
in key_list by entries on the `.aux' file read by the
\begin{document} command.
The optional text argument will appear after the citation,
i.e. \cite[p. 2]{knuth} might produce `[Knuth, p. 2]'.