\cite

Synopsis:

\cite{keys}

or

\cite[subcite]{keys}

Generate as output a citation to the references associated with keys. The mandatory keys is a citation key, or a comma-separated list of citation keys (see \bibitem).

This

The ultimate source is \cite{texbook}.
  ...
\begin{thebibliography}
\bibitem{texbook} 
  Donald Ervin Knuth. 
  \textit{The \TeX book}. 
  Addison-Wesley, Reading, Massachusetts, 1983.
\end{thebibliography}

produces output like ‘... source is [1]’. You can change the appearance of the citation and of the reference by using bibliography styles if you generate automatically the thebibliography environment. More information in Using BibTeX.

The optional argument subcite is appended to the citation. For example, See 14.3 in \cite[p.~314]{texbook} might produce ‘See 14.3 in [1, p. 314]’.

In addition to what appears in the output, \cite writes information to the auxiliary file jobname.aux (see Jobname). For instance, \cite{latexdps} writes ‘\citation{latexdps}’ to that file. This information is used by BibTeX to include in your reference list only those works that you have actually cited; see \nocite also.

If keys is not in your bibliography information then you get ‘LaTeX Warning: There were undefined references’, and in the output the citation shows as a boldface question mark between square brackets. There are two possible causes. If you have mistyped something, as in \cite{texbok} then you need to correct the spelling. On the other hand, if you have just added or modified the bibliographic information and so changed the .aux file (see \bibitem) then the fix may be to run LaTeX again.