%%
%% The LaTeX Companion, 3ed
%%
%% Example A-1-9 on page II-630 in "Defining simple environments".
%%
%% 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{207.0pt}
\newcounter{Citctr} \newsavebox{\Citname}
\newenvironment{Citation}[1]
   {\sbox\Citname{\emph{#1}}%
    \stepcounter{Citctr}\begin{description}
      \item[Citation \arabic{Citctr}]}
   {\hspace*{\fill}\nolinebreak[1]%
    \quad\hspace*{\fill}%
    %% \finalhyphendemerits=0     %% see explanation below
    \usebox{\Citname}\end{description}}
%StartShownPreambleCommands

%StopShownPreambleCommands

\begin{document}
\begin{Citation}{Protagoras}
  Man is the  measure of all things.
\end{Citation}
This is some regular text in between two
Citation environments.
\begin{Citation}{Blaise Pascal}
  On mourra seul.
\end{Citation}
More regular text \ldots
\begin{Citation}{William Pitt}
  Necessity is the plea for every
  infringement of human freedom.
\end{Citation}
\end{document}