%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-1-44 on page I-288 in "thmtools --- Advanced theorem declarations".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{141.0pt}

% FAKE: definition for \cite to fit book bibliography !!!
\renewcommand\cite[1]{[48]}
% FAKE: in real life this example should have a bibliography!!!

%StartShownPreambleCommands
\usepackage{amsthm,thmtools}
\declaretheoremstyle[spaceabove=3pt, spacebelow=3pt,
  headindent=\parindent, headpunct=., headformat=\NOTE,
  notebraces={}{}, notefont=\bfseries, bodyfont=\itshape,
  numbered=no, qed=\qedsymbol ]{citing}
\declaretheorem[style=citing]{varthm}
%StopShownPreambleCommands

\begin{document}
\begin{varthm}[Theorem 3.16 in \cite{Knuth90}]
  By focusing on small details, it is possible to
  understand the deeper significance of a passage.
\end{varthm}
\end{document}